San Manuel’s Integrated Processing Complex: A Record of Copper Recovery in Arizona

A look at the historical recovery methods at the San Manuel Property, a closed copper mine in Arizona, covering its sulfide concentrator, heap leach, in situ leaching, and SX-EW operations.

The San Manuel Property is a historical mine site located in Pinal County, Arizona, approximately 65 miles northeast of Tucson and 75 miles east-southeast of Phoenix. The property, held by Faraday Copper Corp., is not an operating mine. It is a closed and reclaimed former copper producer that operated historically until approximately the late 1990s, with processing facilities on care and maintenance by June 1999, formally closed in October 2003, and demolished by 2007. The property encompasses patented and fee land, unpatented federal mining claims, Arizona State Land Department Mineral Exploration Permits, and split estate surface lands and mineral rights. Its target commodity is copper, with gold, molybdenum, and minor silver historically recovered as by-products. The San Manuel mine was mined via open pit, block cave, and in situ recovery methods over its operating life. A recommended work program to generate a current NI 43-101 compliant Mineral Resource is anticipated to require approximately 82,000 ft of confirmation drilling at an estimated cost of US$17,200,000.

Critical Data

Overview

The historical processing complex at San Manuel comprised two distinct operational units. A conventional crush-grind-flotation concentrator with downstream Mo recovery, smelting, and electrolytic refining treated sulfide ore from 1956 to 1999. A heap-leach, in situ-leach, and SX-EW circuit treated oxide and secondary-sulfide ore from 1986 to 2002. Both units operated in parallel from 1986 onward, sharing on-site sulfuric acid supply from the smelter SO₂ off-gas. The mill, smelter, and electrolytic refinery were integrated facilities on the San Manuel Plant Site itself, with concentrate and intermediate streams transferred between them by short on-site rail and conveyors.

The deposit is one of the largest Laramide porphyry copper systems in Arizona. Mid-Tertiary extension on the low-angle, southwest-dipping San Manuel fault offset the original porphyry column by approximately 3,609 to 4,265 ft of dip-slip, producing two spatially separated orebody segments. The San Manuel deposit extends approximately 7,710 ft down dip and 2,707 ft along strike from near surface at a 0.5% Cu cut-off, while the Kalamazoo deposit extends approximately 6,693 ft down dip and 3,002 ft along strike from a top-of-mineralization depth of approximately 1,600 ft.

Key Process Stages

The sulfide concentrator processed underground block cave feed through a two-stage crushing circuit. Primary crushing was performed at the surface by four Nordberg gyratory crushers each rated at 1,100 mt/h, located adjacent to the production shafts. ROM crushed ore was hauled by rail approximately 6.8 miles to the on-site concentrator. Secondary crushing at the concentrator was performed by 11 Nordberg Symons cone crushers. Grinding consisted of 13 wet grinding sections, each comprised of one rod mill in open circuit plus two ball mills in closed circuit with hydrocyclones.

The flotation circuit employed conventional bulk sulfide flotation followed by selective copper-molybdenum separation. Hydrocyclone overflow fed bulk Cu-Mo rougher flotation where rougher concentrate was reground to 75% to 80% passing 325-mesh and cleaned in conventional cells through 1985. By 1991 to 1992 the cleaner circuit had been reconfigured to use 16 Magma-designed column flotation cells, each 5.9-ft diameter by 39.0-ft high. Chadwick reported eight independent flotation sections with rougher cells comprising ten Maxwell 2,000-ft³ and 143 Wemco 300-ft³ in service.

A separate on-site slag retreatment circuit treated approximately 2,200 mt/d of flash-furnace and converter slag at approximately 3% Cu feed, returning slag concentrate at approximately 30% Cu to the smelter and slag tailings at 0.45% Cu against a target of 0.30%.

The Mo recovery plant treated bulk Cu-Mo concentrate in a selective separation circuit using reagents to depress copper sulfides. The Mo flotation circuit used three successive Cu-depressant chemistries over its operating life: NaOCl-ferrocyanide from January 1956 to January 1964, H₂O₂-ferrocyanide from January 1964 to January 1980, and NaHS plus NaCN from January 1980 onward. The 1985 plant had two parallel flowsheets of identical design. Bulk Cu-Mo concentrate was thickened in a 125-ft Eimco thickener to 60% to 64% solids, held in a holding tank with 7-h retention, conditioned with NaCN plus NaHS dosed at 44% to 46% solids and 20 min retention, then floated through an 8-stage countercurrent cleaning train. A column flotation cell was installed in 1981 for fine-MoS₂ scavenger duty and by March 1985 had been repositioned to replace cleaners 2 to 4.

The smelter and electrolytic refinery operated continuously from January 1956 commissioning to October 2003 final closure. The smelter began with one green-feed reverberatory furnace, three Pierce-Smith converters, two anode vessels, and one anode casting wheel. In July 1988, under an EPA-Magma consent decree, the three reverberatory furnaces were replaced by a single 2,700 to 3,000 mtpd Outokumpu single-burner flash furnace. The first flash-furnace campaign ran for 10 years 9 months 23 days, a world-record campaign for an Outokumpu copper flash furnace, processing approximately 10.9 Mt of new sulfide concentrate yielding 3.1 Mt of new anode copper. The refinery operated electro-refining at 27 A/ft² with a 12-day plating cycle and 350-lb cathodes.

The heap leach facility treated copper oxide ore and low-grade secondary sulfide material that was not amenable to, or uneconomic for, flotation treatment. Six phases of leach pad construction occurred from April 1986 to August 1992, bringing the cumulative pad area to approximately 10,322,600 ft² of HDPE-lined leach surface. The original 1985 to 1986 build was the first plastic-lined heap leach in Arizona. All ore sent to the heap leach was run-of-mine oxide ore at 90% passing 4-inch. Crushing and agglomeration were not employed at any stage of the heap-leach circuit.

The in situ leaching operation ran in two parallel modes for most of its operating life. Surface-to-underground gravity-flow leaching, commissioned June 1986, injected acidified raffinate from the surface into partially rubblized supergene rock above the abandoned San Manuel block-cave underground levels and gravity-drained PLS into adapted underground haulage drifts. Well-to-well leaching, added September 1989, operated in modified seven-spot patterns on open-pit catch benches with 40.0 ft corner-to-corner spacing through 1989 to 1998.

The solvent extraction circuit processed PLS to selectively extract copper using an organic extractant, producing a high-purity copper-loaded electrolyte for the electrowinning circuit and a raffinate solution returned to the leach pads. The EW circuit deposited copper from the rich electrolyte onto copper starter sheets to produce LME Grade A copper cathodes. The SX-EW plant was commissioned in May 1986 with rated cathode capacity of 25,000 st/y at 20 A/ft² but was expanded in 1988 to 50,000 st/y when the ISL streams were added.

The gold cyanidation plant processed Mo-plant final concentrate to recover Au, Ag, and 20% to 30% of the contained Cu. In June 1974, the plant processed 12.2 st/d of Mo concentrate at 0.462 oz Au/short ton, achieving 87% Au extraction by assay and 95% by independent gold balance. Annualized Au production was approximately 1,950 oz/y.

Additional Interesting Data and Summary

Historical mill capacity evolved from 30,000 stpd in September 1955 to a peak rated capacity of 66,000 stpd in 1982. In 1991 and H1 1992 average operating throughput was approximately 55,685 stpd, running at approximately 89% of nameplate. Life-of-mine ore milled totaled 703,686,000 st of sulfide ore at an average feed of 0.66% Cu and approximately 0.03% MoS₂.

The concentrator processed two parallel feed streams: the ROM ore stream at approximately 52,000 mt/d at 0.63% Cu feed grade, and the slag retreatment circuit at 2,200 mt/d of smelter slag at approximately 3% Cu feed. The ROM final flotation tailings grade reached a low of 0.076% Cu in 1990. A ROM head-to-tails balance does not reconcile to the 89.5% recovery figure because that value also captures Cu recovered from smelter slag and returned to smelter feed.

For the heap leach, cumulative ASCu leach extraction from ore into PLS progressed from 73% around 1993 to 88.3% at the end of leaching in 2002. The 1995 to 2002 increase was achieved through residual leaching with controlled surface, side slope, and berm-walled sprinkler-cell irrigation described as rice-paddy irrigation. The LoM heap cathode production was approximately 345,000 st. A 35-ppm cationic fluorochemical surfactant was added to the barren leach during preconditioning from June 1992 onward as standard practice, following a test program that demonstrated cumulative ASCu extraction above 73% with the surfactant versus 63% without.

For in situ leaching, Sutton reconciled the BHP block-model ASCu against cathode-equivalent Cu produced for Zone 6, resulting in cumulative ASCu extraction at zone closure of 41.7%, against a BHP 1999 internal forecast of 52%. The difference was attributed to faulting, block-cave-induced ground subsidence, channeling, and gypsum precipitation in fractures. Sutton revised the estimate to 55% sweep times 90% leachability equals 50% ASCu extraction with continued operation. A Resource Recovery Test from 1992 to 1993, a controlled 551-day well-to-well test, produced 1 million kg of Cu where ASCu extraction was 45.5% in the footwall cell to 64.6% in the hanging-wall cell.

The SX-EW plant achieved COMEX certification in August 1991 and LME Grade A listing in April 1992, the second SX-EW product ever LME-certified. Peak annual production was 124 Mlb in 1993 from heap plus ISL feed. The theoretical maximum at full rectifier capacity and 93% current efficiency was 143 Mlb/y.

Key Processes

• Sulfide ore processing: two-stage crushing, rod and ball mill grinding, bulk Cu-Mo flotation, Mo recovery, smelting, and electrolytic refining.
• Oxide ore processing: run-of-mine heap leaching with dilute sulfuric acid, no crushing or agglomeration.
• Secondary sulfide processing: in situ leaching through surface-to-underground gravity flow and well-to-well modes.
• Solution processing: solvent extraction with oxime-based extractant followed by electrowinning to LME Grade A cathode.
• By-product recovery: gold cyanidation plant processing Mo-plant final concentrate.
• On-site sulfuric acid production from smelter SO₂ off-gas supplying both market sales and SX-EW raffinate make-up.

Source: Technical Report San Manuel Property Pinal County, Arizona, July 29, 2026. Project website: San Manuel Property