The San Manuel Property in Arizona once hosted a fully integrated copper recovery complex, combining sulfide flotation, smelting, and refining with heap and in situ leaching served by a central SX-EW plant. This article reviews the process configuration, equipment, and historical performance of that operation.
Opening Paragraph
The San Manuel Property is a historical mine site located in Pinal County, Arizona, approximately 65 miles northeast of Tucson. Owned by Faraday Copper Corp., the property is not an operating mine and currently holds no current mineral resource or reserve estimates. Historical production of copper, along with by-product molybdenum, gold, and silver, occurred from 1956 to 1999 via underground block-cave mining, open-pit mining, and in situ recovery methods. The property was placed on care and maintenance in 1999, formally closed in 2003, and surface facilities were demolished by 2007. The project is in a closed and reclaimed state, and a recommended work program to generate a current mineral resource is anticipated to require confirmation drilling.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Property Status | Historical mine site | Not applicable | Closed and reclaimed; formally closed in 2003. |
| Commodity | Copper | Not applicable | Molybdenum, gold, and silver were historical by-products. |
| Sulfide Concentrator Operating Period | 1956 to 1999 | Year | Conventional crush-grind-flotation circuit. |
| Heap Leach / SX-EW Operating Period | 1986 to 2002 | Year | Treated oxide and secondary-sulfide ore. |
| LoM Sulfide Ore Milled | 703,686,000 | st | Average feed grade of 0.66% Cu and approximately 0.03% MoS₂. |
| LoM Refined Copper Production | 4,651,600 | st | Produced from 1956 to 1999. |
| LoM Molybdenum in Concentrate | 73,200 | st | Shipped off-site for processing. |
| Design Mill Capacity (Circa 1982) | 66,000 | stpd | Capacity varied over life, from 30,000 stpd in 1955 to 62,500 stpd in 1991. |
| Historical Cu Plant-Circuit Recovery | 89.5 | % | Reported for 1991; includes Cu recovered from smelter slag. |
| LoM Heap Leach ASCu Extraction | 88.3 | % | Achieved at closure in 2002. |
| Historical ISL Zone 6 ASCu Extraction | 41.7 | % | Reported at zone closure. |
| SX-EW Plant Design Capacity (1988) | 100 | Mlb/y | Two tankhouses, each originally designed for 25,000 st/y. |
| SX-EW Peak Annual Production (1993) | 124 | Mlb | Combined heap and ISL feed. |
Overview
The historical processing complex at San Manuel comprised two distinct operational units. The first was a conventional crush-grind-flotation concentrator with downstream molybdenum recovery, smelting, and electrolytic refining that treated sulfide ore from 1956 to 1999. The second was a heap-leach and in situ-leach circuit with a central solvent extraction and electrowinning (SX-EW) plant that treated oxide and secondary-sulfide ore from 1986 to 2002. Both units operated in parallel from 1986 onward, sharing an on-site sulfuric acid supply generated from the smelter’s SO₂ off-gas. The mill, smelter, and electrolytic refinery were integrated facilities on the plant site itself, with concentrate and intermediate streams transferred between them by short on-site rail and conveyors.
The sulfide concentrator processed 703,686,000 st of ore over its 44-year life, producing 4,651,600 st of refined copper. The heap leach and SX-EW circuit contributed additional copper production, with a cumulative acid-soluble copper (ASCu) extraction of 88.3% from stacked ore by the end of leaching in 2002. In situ leaching (ISL) operated in two parallel modes for most of its life, with documented extraction rates that varied by zone.
Key Process Stages
Sulfide Concentrator
The concentrator had 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 then hauled by rail approximately 6.8 miles to the on-site concentrator. Secondary crushing was performed by 11 Nordberg Symons cone crushers.
Grinding consisted of 13 wet grinding sections. Each section comprised one rod mill in open circuit plus two ball mills in closed circuit with hydrocyclones. The smaller sections used 10.0 × 10.0-ft rod mills and 10.0 × 13.0-ft ball mills, while the three large sections used 12.5 × 14.0-ft rod mills and 12.5 × 16.0-ft ball mills. Hydrocyclones were 20.0-in Krebs D20B units in 1982, later upgraded to 26.0-in Krebs units in the late 1980s to early 1990s.
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 in diameter and 39.0-ft high. At that time, the circuit comprised eight independent flotation sections with rougher cells including ten Maxwell 2,000-ft³ and 143 Wemco 300-ft³ cells.
The bulk Cu-Mo cleaner concentrate, which served as feed to the on-site molybdenum recovery plant, contained approximately 30% Cu and 0.90% to 1.00% MoS₂. The final cleaner concentrate, shipped after Mo separation, contained approximately 30% Cu and 0.90% to 1.00% MoS₂. The rougher Cu concentrate contained 9% to 12% Cu and 0.4% to 0.5% MoS₂.
An 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.
Molybdenum Recovery Plant
The molybdenum recovery plant treated the bulk Cu-Mo concentrate in a selective separation circuit using reagents to depress copper sulfides, allowing molybdenite to be floated into a separate concentrate. The Mo flotation circuit used three successive copper-depressant chemistries over its operating life: NaOCl-ferrocyanide from January 1956 to January 1964, H₂O₂-ferrocyanide from January 1964 to January 1980, and NaHS + NaCN from January 1980 onward.
The process involved thickening the bulk Cu-Mo concentrate in a 125-ft Eimco thickener to 60% to 64% solids, holding it in a 25 × 36.1-ft tank with 7-hour retention, and conditioning it in a 10.0 × 10.0-ft Denver overflow conditioner with NaCN + NaHS dosed at 44% to 46% solids and 20-minute retention. The conditioned pulp then passed through an 8-stage countercurrent cleaning train, using Galigher #36 cells in early stages and Denver #18 Specials in stages 4 to 8. A 4.00-ft square × 32.0-ft column flotation cell was installed in 1981 for fine-MoS₂ scavenger duty and was later repositioned to replace cleaners 2 to 4.
The 8th-cleaner concentrate, containing approximately 87% to 88% MoS₂ and 1.2% to 1.3% Cu, was repulped to 50% solids and held with NaCN at 0.2% free CN for 36 hours before three-stage countercurrent recleaning with attrition scrubbing and sodium-silicate as an "insol" depressant. Final Mo concentrate contained approximately 91% to 92% MoS₂ and 0.75% Cu. The concentrate was filtered, dried in multi-hearth Skinner dryers to less than 10% moisture, and packaged in 55-gallon steel drums for off-site sale.
Smelting and Refining
The on-site smelter operated continuously from January 1956 to October 2003. The original configuration included one green-feed reverberatory furnace, three Pierce-Smith converters, two anode vessels, and one anode casting wheel. Through the late 1960s and early 1970s, it was expanded to three reverberatory furnaces, six converters, four anode vessels, and two casting wheels. A two-train Monsanto sulfuric acid plant was added in 1974.
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, and the acid plant was modified from single-contact to double-absorption. The first flash-furnace campaign ran from July 1988 to May 1999, a world-record campaign for an Outokumpu copper flash furnace, processing approximately 10.9 Mt of new sulfide concentrate and yielding 3.1 Mt of new anode copper. The smelter drew roughly two-thirds of its feed from Magma's San Manuel, Pinto Valley, and Superior mines, with one-third from open-market toll smelting.
The refinery operated electro-refining at 27 A/ft² with a 12-day plating cycle and produced 350-lb cathodes. A 1995 bismuth quality issue in cathodes was managed by operational changes to starter-sheet production and electrolyte bleed.
Heap Leaching
A run-of-mine leach facility treated copper oxide ore and low-grade secondary sulfide material that was not amenable to flotation. The operation used dilute sulfuric acid solutions applied directly to ore. 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 at 90% passing 4-inch. Crushing and agglomeration were not employed. Lift height was 15-ft from 1986 through early 1992, reduced to 10-ft thereafter, which drove an increase of approximately 5% in cumulative recovery. The final stack configuration in 1999 included 18 leach modules, each approximately 400 × 300 ft, with an ultimate stack height of about 300 ft and a 30° side slope.
Standard pre-cure practice involved two sulfuric acid applications at 250 g/L: a first application of 15 lb/ton of ore with a 4-day rest, then a second application of 10 lb/ton with a 3-day rest. Operating raffinate was pH 1.5 to 1.6, containing 0.06 to 0.20 g/L Cu, 1.13 to 1.8 g/L Fe, and 15 to 18 g/L free H₂SO₄, applied at 5.15 gal/(h·m²) at active loading and reduced to 1.9 to 2.6 gal/(h·m²) for residual leach. A 35-ppm cationic fluorochemical surfactant was added to the barren leach from June 1992 onward, following a test program that demonstrated cumulative ASCu extraction above 73% with the surfactant versus 63% without.
In Situ Leaching
In situ leaching operated in two modes. Surface-to-underground gravity-flow leaching, commissioned in June 1986 with 26 injection wells, was the sole mode through September 1989. This mode injected acidified raffinate from the surface into partially rubblized supergene rock above the abandoned block-cave levels, with PLS draining by gravity into adapted underground haulage drifts before being pumped approximately 2,380 ft to the surface SX-EW plant. Well-to-well leaching was added in September 1989, operating in modified seven-spot patterns on open-pit catch benches with 40.0-ft corner-to-corner spacing, later expanded to 49.87-ft for the 1998 west-side expansion.
Injection wells were reverse-circulation drilled with 1.5-inch Schedule 80 PVC casing. Production wells were mud-rotary drilled with 6-inch Schedule 80 PVC casing, 328 to 492-ft deep, fitted with submersible 304 SS / 316 SS centrifugal pumps at 7.4 to 11 kW. The active leach zone was 200 to 394 ft below the bench surface. By late 1998, the operation comprised approximately 1,200 wells across six leaching zones.
Solvent Extraction and Electrowinning
The SX-EW plant was commissioned in May 1986 with a rated cathode capacity of 25,000 st/y at 20 A/ft², expanded in 1988 to 50,000 st/y when ISL streams were added. The plant baseline included two tankhouses, each with two rectifiers and 94 EW cells. Each tankhouse was designed for 25,000 st/y but achieved 32,500 st/y in operation. Combined PLS feed of approximately 17,611 to 19,813 gpm was collected in a 1,341,600 ft³ plant feed pond before flowing to four parallel SX trains.
The solvent extraction circuit used an oxime-based organic extractant, specifically Henkel LIX 984, in Phillips 85X7 diluent. A semi-automatic montmorillonite-clay treatment system on a Sparkler vertical-plate stainless filter, retrofitted in 1991 to 1992, cleaned circuit organic, recovered entrained organic from the raffinate pond, and increased SX flow capacity from 16,000 to 19,500 gpm. The EW circuit deposited copper onto starter sheets to produce LME Grade A copper cathodes at 99.999% Cu. The San Manuel cathode was COMEX-certified in August 1991 and LME-listed as Grade A Copper in April 1992, the second SX-EW product ever LME-certified.
Gold Cyanidation Plant
A small on-site 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.
Additional Interesting Data and Summary
Historical mill copper recovery was reported at 89.5% in 1991, a figure that captures copper recovered from smelter slag and returned to smelter feed. The ROM final flotation tailings grade reached a low of 0.076% Cu in 1990. Molybdenum recovery to the bulk Cu-Mo concentrate was 79.68% in 1984 and 81.91% in 1985 YTD, while recovery from bulk concentrate to final Mo concentrate was 82.75% in 1984 and 65.93% overall that year.
Heap leach cumulative ASCu extraction progressed from 73% around 1993 to 74.5% at the end of open-pit feed in January 1995, then to 86% at the end of 1998, and finally 88.3% at the end of leaching in 2002. For in situ leaching, a reconciliation for Zone 6, the longest-running ISL zone, showed cumulative ASCu extraction of 41.7% at zone closure against a BHP forecast of 52%. A controlled 551-day well-to-well test on a three-cell array produced 1 million kg of Cu, with ASCu extraction ranging from 45.5% in the footwall cell to 64.6% in the hanging-wall cell, demonstrating structural control of sweep efficiency.
The San Manuel operation represents a complete historical record of a large, integrated copper processing complex. All facilities are closed and demolished, but the technical documentation of equipment, flowsheets, and operating performance provides a detailed reference for the processing methods employed at this site.
Key Processes
- Two-stage crushing (gyratory and cone crushers) followed by rod mill and ball mill grinding in closed circuit with hydrocyclones
- Bulk Cu-Mo flotation with regrind and column flotation cells, followed by selective Cu-Mo separation
- Molybdenum recovery via differential flotation with copper sulfide depression using NaHS and NaCN
- Smelting in an Outokumpu flash furnace with double-absorption sulfuric acid plant
- Electrolytic refining at 27 A/ft² producing 350-lb LME Grade A copper cathodes
- Run-of-mine heap leaching with HDPE-lined pads, sprinkler irrigation, and surfactant addition
- In situ leaching via surface-to-underground gravity flow and well-to-well injection patterns
- Solvent extraction using oxime extractant with a clay treatment system for organic recovery
- Electrowinning of high-purity copper cathode
- Gold cyanidation of Mo-plant concentrate for by-product recovery
Source: Technical Report, San Manuel Property, July 29, 2026. Project website: San Manuel Property
