SX-EW Copper Processing Explained: From Oxide Ore to LME Grade Cathode

Most people in the mining industry know that copper comes out of the ground as sulphide ore, gets concentrated, smelted, and electrorefined into 99.99% pure cathode. That pyrometallurgical route has dominated copper production for over a century and still accounts for roughly 80% of global output. But a growing share of the world's copper — particularly from Chile, the DRC, and the southwestern United States — takes a fundamentally different path. Solvent extraction and electrowinning, universally abbreviated as SX-EW, produces LME Grade A copper cathode without a smelter, without a refinery, and without the SO₂ emissions, slag, and energy intensity of the pyrometallurgical route. This article explains how SX-EW works, why it matters to the copper supply chain, and where Sterling Ore's operations fit into the picture.

The Feedstock: Why Oxide Ores Need a Different Route

The starting point for understanding SX-EW is recognising that not all copper ores are created equal. Sulphide ores — chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄), chalcocite (Cu₂S) — contain copper bonded to sulphur. These minerals float beautifully in froth flotation, producing a concentrate of 25–35% copper that can be fed to a smelter. The smelter oxidises the sulphur (producing SO₂, which is captured as sulphuric acid in modern plants) and produces a matte of 50–70% copper, which is further refined to anode copper (99.5%) and then electrorefined to cathode.

Oxide ores — malachite (Cu₂CO₃(OH)₂), azurite (Cu₃(CO₃)₂(OH)₂), chrysocolla ((Cu,Al)₂H₂Si₂O₅(OH)₄·nH₂O), and cuprite (Cu₂O) — are chemically different. Their copper is bonded to oxygen, carbonate, or silicate, not sulphur. They do not respond to flotation. Until the 1960s, copper oxide deposits were essentially stranded assets: the copper was there, often at grades comparable to sulphide deposits, but there was no economically viable way to extract it at scale.

SX-EW solved this problem. It processes oxide ores through a hydrometallurgical route — leaching, solvent extraction, and electrowinning — that produces finished cathode at the mine site, bypassing the smelting and electrorefining stages entirely. The economic and environmental implications are substantial.

Stage One: Leaching — Getting the Copper into Solution

The first stage of SX-EW is leaching: dissolving copper from the ore into an aqueous solution. The dominant method is heap leaching, which is conceptually straightforward but operationally sophisticated.

Crushed oxide ore — typically crushed to minus 19mm (¾ inch), though some operations crush finer — is stacked in lifts of 6–10 metres onto an impermeable liner (usually HDPE geomembrane over compacted clay). The heap is irrigated with a dilute sulphuric acid solution (typically 5–15 g/L H₂SO₄) via drip emitters or sprinklers. As the acid percolates through the heap over a period of months, it dissolves copper-bearing oxide minerals through reactions such as:

Cu₂CO₃(OH)₂ + 2H₂SO₄ → 2CuSO₄ + CO₂ + 3H₂O
(malachite dissolution — the workhorse reaction in heap leaching)

The resulting solution, called pregnant leach solution (PLS), contains 1–6 grams of copper per litre, along with dissolved iron, aluminium, manganese, and other metals that were present in the ore. The PLS is collected at the base of the heap via a network of drainage pipes and directed to a holding pond, from which it is pumped to the solvent extraction plant.

Heap leaching is not fast. Typical leach cycles run 6–18 months, with copper recovery rates of 65–85% depending on ore mineralogy, crush size, acid concentration, and climate (leaching kinetics slow dramatically in cold weather). But the capital cost is a fraction of a concentrator-smelter complex, and the operating cost — typically $1.50–$2.50 per pound of copper produced — is among the lowest in the industry.

An alternative for higher-grade oxide deposits is agitated tank leaching, in which finely ground ore is slurried with acid in stirred tanks. Tank leaching achieves faster kinetics (hours to days rather than months) and recovery rates above 90%, but at higher capital and operating cost. It is typically reserved for ores with grades above 1.5% copper or where the orebody contains significant quantities of acid-consuming gangue minerals (particularly carbonates like calcite and dolomite) that make heap leaching uneconomic.

Stage Two: Solvent Extraction — Purifying and Concentrating

The PLS leaving the heap is a weak, impure copper sulphate solution. Turning it directly into cathode-quality copper would be impossible — the impurities would contaminate the cathode, and the low copper concentration would make electrowinning hopelessly inefficient. Solvent extraction bridges the gap.

SX is a liquid-liquid ion exchange process. The PLS is contacted with an organic phase — a kerosene-based diluent carrying an extractant molecule, typically a modified aldoxime or ketoxime (produced by BASF, Solvay, and other specialty chemical manufacturers under trade names like LIX, Acorga, and Cyanex). The extractant molecule selectively binds copper ions (Cu²⁺) in preference to iron, aluminium, and manganese through a chelation reaction:

Cu²⁺(aq) + 2RH(org) → R₂Cu(org) + 2H⁺(aq)
(copper transfers from the aqueous PLS into the organic phase; RH represents the extractant molecule)

The organic and aqueous phases are mixed vigorously in a series of mixer-settlers — large tanks where the two liquids are dispersed into droplets, providing enormous surface area for mass transfer, then allowed to separate by gravity (the organic phase is lighter and floats to the top). A typical SX circuit has two extraction stages and one stripping stage (an "E-E-S" or "2E-1S" configuration), though larger operations may use three extraction and two stripping stages.

After the extraction stages, the copper-loaded organic phase — now carrying 8–12 g/L Cu — flows to the stripping stage, where it is contacted with a strong acid electrolyte (typically 170–190 g/L H₂SO₄, 35–45 g/L Cu). The high acidity reverses the extraction reaction, stripping the copper from the organic back into the aqueous phase:

R₂Cu(org) + 2H⁺(aq) → Cu²⁺(aq) + 2RH(org)
(copper is stripped from the organic into the electrolyte, regenerating the extractant)

The result of SX is a rich electrolyte — a purified, concentrated copper sulphate/sulphuric acid solution containing 45–50 g/L Cu — that is ready for electrowinning. The stripped organic phase is recycled to the extraction stage. The raffinate (the copper-depleted aqueous phase leaving extraction) is replenished with fresh acid and returned to the heap for more leaching.

The selectivity of modern extractants is remarkable. A well-operated SX plant rejects more than 99% of iron, aluminium, and manganese while transferring copper with efficiency above 95%. The extractants are also selective for copper over cobalt and nickel, which is operationally convenient but means that separate circuits are needed if these metals are to be recovered economically.

Stage Three: Electrowinning — Plating Pure Copper

The final stage, electrowinning, is an electrolytic process that plates metallic copper from the rich electrolyte onto cathodes. It is closely related to electrorefining — the process used to purify smelter-produced anode copper — but with a crucial difference: in electrorefining, the copper source is a dissolving anode (99.5% Cu). In electrowinning, the copper source is the electrolyte itself, and the anode is an inert material (typically a lead alloy) that does not dissolve.

The EW cell contains alternating lead-alloy anodes and stainless-steel cathode blanks suspended in the rich electrolyte. When DC current is applied (typically at 250–350 A/m² current density), the following reactions occur:

At the cathode: Cu²⁺ + 2e⁻ → Cu⁰ (metallic copper plates onto the stainless steel)
At the anode: H₂O → ½O₂ + 2H⁺ + 2e⁻ (oxygen evolution — the inert anode does not dissolve)

Copper plates onto the cathode blanks over a period of 6–8 days, forming plates weighing 45–55 kg each. The cathodes are harvested by automated stripping machines, washed, stacked, and strapped for shipment. The quality is exceptional: SX-EW cathodes typically assay at 99.999% copper, exceeding the 99.99% minimum for LME Grade A registration. The surface finish is smooth, dense, and free of the nodules and dendrites that can form in poorly controlled electrorefining operations.

The spent electrolyte — now depleted to roughly 30–35 g/L Cu and enriched in acid — is returned to the SX stripping stage to collect more copper, closing the circuit. This continuous recirculation is one of the elegant features of SX-EW: the acid generated at the anode is exactly the acid consumed in stripping, so the overall process is acid-neutral.

Why SX-EW Matters: Economics, Environment, and Geography

SX-EW is not a niche process. As of 2025, it accounts for approximately 16–18% of global copper cathode production, or roughly 4 million tonnes per year. Its significance extends well beyond that tonnage figure. Several factors make SX-EW strategically important to the copper supply chain:

Capital Efficiency

A greenfield SX-EW operation — heap leach pads, SX plant, and EW tankhouse — costs roughly $6,000–$10,000 per annual tonne of capacity, compared to $18,000–$25,000 per annual tonne for a concentrator-smelter-refinery complex. This capital efficiency is transformative for mid-tier copper deposits that would not support the investment required for a pyrometallurgical route. Much of the copper production growth in the DRC and Zambia over the past decade has been SX-EW based, precisely because it makes smaller, lower-grade deposits economically viable.

Energy and Emissions

Smelting copper concentrate is energy-intensive. Drying, roasting (if used), smelting to matte, converting to blister, and anode casting consume roughly 8–12 GJ per tonne of copper, mostly from fossil fuels. SX-EW, by contrast, consumes 3–5 GJ per tonne — roughly one-third of the smelting route — and the energy is primarily electricity, which can be sourced from renewables. At operations in Chile's Atacama Desert and the southwestern US, SX-EW plants increasingly run on solar power, driving the carbon intensity of cathode production far below the industry average.

No SO₂, No Slag

Copper smelters produce approximately 2–3 tonnes of SO₂ per tonne of copper from sulphide concentrates. Modern smelters capture 95–99% of this SO₂ and convert it to sulphuric acid — which itself becomes an input to heap leaching, creating a useful industrial symbiosis — but older plants, particularly in jurisdictions with weaker environmental enforcement, release significant volumes. SX-EW produces zero SO₂ emissions and generates no slag (typically 2–3 tonnes per tonne of copper from smelting). For copper buyers with ESG mandates, the SX-EW route offers a materially cleaner supply chain.

Geographic Flexibility

SX-EW plants can be built at the mine site, in remote locations, without the infrastructure requirements of a smelter — no need for concentrate transport, no matte handling, no anode casting and shipping. This is particularly valuable in landlocked copper-producing regions like the African Copperbelt, where the cost and logistics of shipping concentrate to coastal smelters can erode project economics. SX-EW cathode goes straight from the tankhouse to the container, ready for export.

The Limitations: What SX-EW Cannot Do

For all its advantages, SX-EW is not a universal solution. It has specific limitations that define where it can and cannot be applied:

• Ore type restriction: SX-EW works on oxide and secondary sulphide (chalcocite) ores. It does not work on primary sulphides (chalcopyrite), which constitute the majority of the world's remaining copper resources. Chalcopyrite is refractory to acid leaching at ambient temperature and pressure. High-temperature, high-pressure leaching processes (such as the CESL, Dynatec, and Galvanox processes) have been developed for chalcopyrite concentrates, but none have achieved widespread commercial deployment. Until they do, SX-EW will remain limited to the oxide and secondary sulphide portion of copper resources.
• Acid consumption: Heap leaching consumes sulphuric acid. Mines located far from sulphuric acid supply — or processing ores with high carbonate content that neutralise acid — face uneconomic acid costs. The availability of low-cost sulphuric acid (often from nearby smelters) is a critical site-selection factor for SX-EW operations.
• Water requirement: While SX-EW uses less energy than smelting, it uses more water. Heap leaching, SX, and EW together consume roughly 3–5 m³ of water per tonne of ore processed — manageable in temperate regions but a significant constraint in the hyper-arid Atacama, where most of the world's largest SX-EW operations are located. Many Chilean operations now use desalinated seawater pumped from the coast, adding to both capital and operating cost.
• Organic losses: The SX organic phase is expensive (roughly $5–8 per litre for the extractant-diluent mixture) and is gradually lost through entrainment (tiny droplets carried out with the aqueous streams), degradation (hydrolysis of the extractant molecule under acidic conditions), and crud formation (a stable emulsion that accumulates at the organic-aqueous interface). Loss rates of 50–200 ppm (litres of organic lost per tonne of copper produced) are typical, adding $30–$120 per tonne of copper to operating cost.

Sterling Ore's SX-EW Operations: Mopani Ridge

Sterling Ore Solutions operates a 25,000 tonnes-per-annum SX-EW facility at our Mopani Ridge copper complex, processing oxide ore from open-pit operations with an average head grade of 0.85% copper. The heap leach pads cover 180 hectares and contain approximately 12 million tonnes of ore under active irrigation at any given time.

Our SX circuit uses a three-extraction, two-strip configuration with a modified ketoxime-aldoxime blend optimised for the specific impurity profile of Mopani Ridge PLS, which is characterised by elevated iron (2–4 g/L) and silica (0.5–1.0 g/L). The EW tankhouse contains 144 cells with 60 stainless-steel cathodes each, operating at 310 A/m² and producing cathodes that consistently assay 99.999% Cu.

The Mopani Ridge SX-EW plant is integrated with our closed-loop water treatment system (HRRO), which recovers process water from the SX raffinate stream and reduces raw water intake — a subject we covered in detail in our article on water stewardship in mining operations. The combination of SX-EW with advanced water treatment gives Mopani Ridge one of the lowest environmental footprints per tonne of copper produced of any operation on the African Copperbelt.

The Future: SX-EW's Place in the Copper Supply Chain

The long-term outlook for SX-EW is tied to two variables: the availability of oxide copper resources and the development of commercially viable hydrometallurgical processes for primary sulphides.

On the resource side, known oxide copper deposits are being depleted faster than new ones are being discovered. The giant Chilean SX-EW operations — Escondida, Chuquicamata, Radomiro Tomic — are gradually transitioning from oxide to sulphide ores as their near-surface oxide caps are exhausted, requiring the construction of concentrators where SX-EW plants once stood. This transition is a significant driver of the copper industry's rising capital intensity.

On the technology side, the holy grail remains commercial-scale leaching of chalcopyrite. If a process can be developed that leaches chalcopyrite concentrates or run-of-mine ore economically — solving the passivation problem (a layer of elemental sulphur and iron compounds that forms on chalcopyrite particles during leaching, blocking further reaction) — it would dramatically expand the share of copper resources amenable to SX-EW. Multiple technology companies (FLSmidth, Jetti Resources, and Anglo American, among others) are pursuing this, with pilot and demonstration plants in operation, but none has yet achieved widespread commercial deployment at the scale needed to displace smelting.

In the meantime, SX-EW's share of global copper production is likely to remain stable at 16–20%, with growth in Africa (DRC, Zambia) offsetting the gradual decline of oxide operations in Chile. For institutional investors in copper, understanding SX-EW is essential — not because it will replace smelting, but because the projects that use it have fundamentally different capital intensity, operating cost structures, environmental profiles, and risk factors than the pyrometallurgical projects that dominate market commentary.

The Bottom Line

SX-EW is the most important copper processing technology that most investors have never heard of. It produces roughly one-sixth of the world's copper cathode — LME Grade A quality, 99.999% pure — without a smelter, without SO₂ emissions, and at roughly one-third the capital cost and energy intensity of the pyrometallurgical route. It is the reason oxide copper deposits are economically viable, and it is the technology underpinning the rapid growth of African copper production.

Its limitations are real: it only works on oxides and secondary sulphides, it consumes large volumes of acid and water, and it cannot (yet) economically process the chalcopyrite ores that dominate global copper resources. But within its domain — and that domain is measured in millions of tonnes per year — SX-EW is the benchmark against which other processing routes are measured.

For institutional buyers sourcing LME Grade A cathode, an SX-EW cathode and a smelter-produced electrorefined cathode are chemically and physically identical — the LME specification does not distinguish between production routes. For investors evaluating copper projects, understanding which route a deposit will take is fundamental to assessing its capital requirements, operating costs, environmental permitting pathway, and overall risk profile. The difference between a $1.5 billion SX-EW mine and a $4 billion concentrator-smelter complex is the difference between a project that gets financed and one that does not.