Inside the Cactus Mine: A Deep Dive into the 28 Mtpa Heap Leach SX-EW Copper Processing Circuit

Overview

Nestled in the mineral-rich landscape of Pinal County, Arizona, the Cactus Mine Project stands as a significant development in the North American copper industry. Operated by the Arizona Sonoran Copper Company Inc. (ASCU), this project aims to revitalize a historic mining district near Casa Grande into a major, modern producer of high-purity copper cathode. The project’s significance is underscored by a comprehensive Pre-Feasibility Study (PFS) detailed in an NI 43-101 Technical Report, dated October 2025, which outlines a robust plan for sustainable copper extraction. The Cactus Mine is designed to process a substantial 28 million dry tons of ore annually from two primary open pits—Parks/Salyer and Cactus West—which contain a mix of oxide, enriched, and primary sulfide copper mineralization. Unlike traditional concentrator-based operations, Cactus will employ a large-scale heap leach, solvent extraction, and electrowinning (HL-SX-EW) circuit, a hydrometallurgical method renowned for its lower energy intensity and cost-effectiveness for certain ore types. This strategic choice positions the project to be a competitive supplier of LME Grade A copper cathode, a critical material for the global energy transition, including electric vehicles and renewable infrastructure. The planned phased expansion, doubling cathode production from 70,000 to 140,000 tons per year, highlights the project’s scale and long-term vision within Arizona’s established and geopolitically stable mining jurisdiction.

Key Process Stages

The Cactus Mine mineral processing circuit is a classic example of modern hydrometallurgy applied to copper extraction. The flowsheet is elegantly straightforward yet engineered for high efficiency and reliability, moving from coarse rock to finished cathode through a series of integrated stages. The design prioritizes robust equipment selection and operational flexibility to handle the projected feed over a 20-year mine life.

• Stage 1: Primary & Secondary Crushing: Run-of-Mine (ROM) ore, with a maximum size of 24 inches, is first reduced by a high-capacity primary sizer crusher operating at 4,298 dry tons per hour. The crushed material is then screened. The undersize is conveyed directly to load-out, while the oversize reports to the secondary crushing circuit. This circuit features four standard cone crushers and four double-deck vibrating screens in a closed configuration to ensure a tightly controlled final product size. The target is an 80% passing (P80) size of 3/4 inch (19 mm), which optimizes permeability and leach kinetics in the subsequent heap leach.
• Stage 2: Heap Leaching: The crushed ore is trucked to a lined Heap Leach Pad (HLP). Material is placed in 30-foot lifts, with a maximum ultimate heap height of 250 feet. A raffinate solution—a dilute sulfuric acid—is irrigated over the heap via buried drip emitters to minimize evaporation. As the solution percolates through the ore, it dissolves the copper. Critical to the process is the injection of air via blowers at the base of every second lift to ensure sufficient oxygen is present for the leaching reactions, especially for sulfide minerals. The copper-laden solution, now called Pregnant Leach Solution (PLS), drains by gravity to a collection pond.
• Stage 3: Solvent Extraction (SX): The PLS is pumped to the SX plant at a design flow rate of 21,800 gallons per minute. Here, copper is selectively transferred from the aqueous PLS into an organic extractant solution through a series of mixer-settlers. The plant is designed to operate in a series-parallel configuration with three extraction stages and one stripping stage. This process purifies and concentrates the copper, producing a “loaded organic” stream and a depleted aqueous stream (raffinate). The raffinate is re-acidified and recirculated back to the heap leach, creating a closed-loop solution management system.
• Stage 4: Electrowinning (EW): In the electrowinning tankhouse, copper is plated onto stainless steel cathode blanks from the rich electrolyte derived from the SX strip stage. The initial circuit comprises 102 cells, each containing 85 cathodes and 84 lead-alloy anodes, operating at a current density of 29 amps per square foot. A second, identical circuit will be added between years 3 and 4 to double capacity. Additives like cobalt sulfate (to control anode corrosion) and guar (a smoothing agent) are used to ensure the production of high-quality, market-grade cathode. The final product is a 99.99% pure copper cathode, stripped mechanically, bundled, and prepared for shipment.
• Stage 5: Reagent & Utility Systems: This supporting stage is critical for operations. It includes the bulk storage and dosing of concentrated sulfuric acid (average 253 tons/day), management of SX reagents (extractant, diluent), and EW additives. The circuit also integrates comprehensive water management, drawing on a secured supply from existing rights and pit dewatering, and a significant power distribution network to drive the entire 59.7 MW process plant.

Critical Data

The following table summarizes the key technical and operational parameters that define the scale and efficiency of the Cactus Mine processing circuit, as outlined in the 2025 Pre-Feasibility Study.

Additional Interesting Data and Summary

The Cactus Mine Project is not only defined by its impressive throughput but also by the detailed engineering and sustainability considerations embedded in its design. The metallurgical projection forecasts a variable but strong production profile, reaching the full 140,000-ton-per-year cathode capacity by Year 5 and sustaining it through periods of higher-grade ore. The average feed grade over the first 15 years is a respectable 0.605% CuT, with a life-of-mine average of 0.517% CuT. A key efficiency metric is the net acid consumption, which is projected at a low 0.9 pounds of sulfuric acid per pound of copper produced over the LOM, reflecting favorable ore mineralogy and efficient leaching conditions.

From an environmental and operational standpoint, the project incorporates several notable features. Water consumption is carefully modeled on a monthly basis, with peak usage during the hot summer months (e.g., ~4.1 million gallons per day in July). The use of buried drip irrigation on the heap leach pad is a specific design choice to drastically reduce evaporative losses—a critical consideration in arid Arizona. The connected power demand of nearly 60 MW highlights the scale of the operation, yet the HL-SX-EW process is generally less energy-intensive per pound of copper than conventional milling and smelting. The report also details comprehensive reagent consumption rates, from SX extractant (0.34 gallons per ton of copper) to EW additives like cobalt sulfate.

Economically, the project represents a major investment in Arizona’s copper belt, with the phased expansion providing a pathway to scale production responsibly. The use of the HL-SX-EW method allows for a smaller physical footprint for the processing plant compared to a concentrator and eliminates the need for a tailings storage facility, as the leached ore (spent heap) remains stacked on the lined pad. This technical approach, combined with the project’s location in a mining-friendly jurisdiction with established infrastructure, positions the Cactus Mine as a strategically important, future-ready source of essential copper for the global market.

Source: NI 43-101 Technical Report | Pre-Feasibility Study | Project: Cactus Mine Project | Date: October 2025

Further Reading

• Cactus Mine Project
• Cactus Mine, Arizona