Copper SX/EW runs on clean electrolyte — and a quiet bleed that saves current

In copper solvent extraction/electrowinning — SX/EW (solvent extraction moves copper into an organic phase; electrowinning plates metal from an acid electrolyte) — the numbers are unforgiving. Jarosite precipitation at about 95 °C and pH≈2 can remove roughly 70% of iron from copper raffinate (the aqueous phase left after extraction), while simple Fe(OH)₃ precipitation at the same pH removes only about 20% (www.researchgate.net). Let too much Fe slip through and electrowinning (EW) current efficiency can collapse from about 99.6% to about 38.2% as ferric ions cycle at the electrodes (www.researchgate.net).

Operators fight back with chemistry, separation, and circuit design. The SX section acts as the front‑line impurity filter; precipitation, ion exchange, and solid–liquid separation polish what remains; and a routine bleed removes the impurities SX doesn’t catch. Each piece is tied to hard targets — think Cu:Fe selectivity of ~100–200:1 in extraction, chloride well below 50 mg/L, and Fe held near ~3 g/L in the electrolyte via bleed — documented by plant practice, literature, and patents (www.911metallurgist.com; blog.emew.com).

Chemical precipitation and polishing steps

Chemical precipitation is the primary clean‑up before EW: raising pH (e.g., NaOH, often with an oxidant) drops Fe and other metals as solids. At ≈95 °C/pH≈2, jarosite precipitation removes about 70% of Fe from industrial copper raffinate, versus only about 20% for Fe(OH)₃ at the same pH (www.researchgate.net). Diluting the solution — akin to bleed makeup — increased iron removal efficiency by up to 150% in tests (www.researchgate.net). For accurate caustic or oxidant addition, plants commonly deploy a dosing pump to hold tight pH and redox setpoints.

Other purification chemistries target specific species: Zn‑dust cementation at pH~5 removes trace Cu, Al, and Cd, while sulfide additions (Na₂S, H₂S) precipitate Pb, Ag, and similar ions (link.springer.com). Kaya (2023) reports that two‑step precipitation (e.g., hydroxide followed by Zn‑dust or sulfide) yields higher removals than a single batch (link.springer.com).

Ion exchange is the next polishing lever. In one study, Purolite S957 resin showed a capacity of approximately 42 mg Fe per gram to adsorb residual Fe from dilute electrolyte (www.researchgate.net). Plants implement this with ion exchange systems and appropriately selected ion‑exchange resin beds for Fe scavenging.

Solid–liquid separation closes the loop: filtration or settling removes slimes and colloids generated during precipitation. Facilities routinely specify a clarifier for thickening and a final polishing step via a cartridge filter. In practice, controlled precipitation has yielded thickener/precipitate outputs with more than 66% Fe removal and less than 1% Cl carryover, and well‑seeded Fe hydroxides have been converted to hematite solids with greater than 66% Fe content and less than 1% Cl (www.researchgate.net). Hematite‑seed precipitation in particular produced a solid product with more than 66% Fe (www.researchgate.net).

Example data reinforce the point: Izadi et al. (2017) reported jarosite precipitation (pH 2 at 95 °C) removed about 70% of iron from industrial copper raffinate, versus only about 20% with hydroxide at the same pH, and found that diluting the solution increased iron removal efficiency by up to 150% (www.researchgate.net). These figures underscore that proper chemistry can scrub most impurity metals from the electrolyte.

Solvent extraction as impurity filter

The SX section is the front‑line filter. A copper‑selective oxime (e.g., LIX‑series) pulls Cu²⁺ into the organic phase and leaves most other cations (Fe, Al, Ni, etc.) in the raffinate. As Monhemius observed, SX can theoretically produce an organic phase “containing only the ions of interest, with the complete absence of other inorganic ions” (www.911metallurgist.com). In practice, selectivity is finite: for common extractants (LIX‑64N), reported Cu:Fe extraction ratios range from approximately 200:1 in ideal conditions down to less than 100:1 in poor cases (www.911metallurgist.com).

The stakes are high. Guidance is to keep Cu:Fe (atomic) above 6:1 to avoid parasitic iron redox losses (blog.emew.com). When Fe³⁺ enters the EW strip solution, it cycles at the electrodes, consumes current, and can even dissolve deposited copper (www.911metallurgist.com). Yang and Wang (2023) quantified the effect: raising Fe³⁺ in the catholyte cut first‑stage EW current efficiency from about 99.6% to about 38.2% (www.researchgate.net).

Design choices — extractant strength, scrubbing stages, organic:aqueous ratios — are made to maximize copper selectivity and minimize iron carryover. In practical terms, a healthy SX circuit delivers approximately 30 g/L Cu to EW with only a few grams per liter of Fe (www.911metallurgist.com). Other impurities must be managed upstream: chloride and fluorides cannot be removed by SX and must be held very low (≪50 mg/L Cl⁻, ≪5 mg/L F⁻/Br⁻) to avoid corrosion and halogen evolution (blog.emew.com; blog.emew.com).

Key figures reinforce the operating window. Commercial LIX extractants show Cu:Fe selectivity around 100–200:1 (www.911metallurgist.com); at 100:1, roughly 1% of the transferred copper would be accompanied by iron into the EW feed (www.911metallurgist.com). In a typical design, the “strong electrolyte” contains ~30 g/L Cu and about 3 g/L Fe — a “manageable” level that minimizes impurity co‑deposition — achieved by the SX copper/iron selectivity (www.911metallurgist.com; www.researchgate.net).

Electrolyte bleed and impurity control

Even with SX purification, trace impurities accumulate in the circulating EW electrolyte. The industry answer is a routine bleed: removing a portion of lean electrolyte for treatment and replacing it with clean sulfuric acid and water (patents.justia.com). Patents explicitly note that “iron may build up in concentration in the electrowinning station… Thus, a bleed stream may be taken for treatment” (patents.google.com).

In practice, plants set bleed rates to hold key impurities constant. For copper plating, operators typically target about 3 g/L Fe in the electrolyte by bleeding; at around 30 g/L Cu in the circuit, bleed is used to keep Fe near that ~3 g/L “manageable” level (www.911metallurgist.com). Quantitatively, if the SX Cu:Fe ratio is ~100:1, roughly 10% of the copper entering EW ends up being bled off with the spent solution (www.911metallurgist.com). The bleed is then processed by filtration or precipitation — jarosite or hydroxide — to remove Fe and other metals; facilities often use acid‑resistant housings such as a steel filter for this duty.

One design (100 t/d Cu) used a crystallizer on the bleed to recover copper and discard an Fe‑rich solution; this cut copper lost in bleed to about 4% of SX output (from about 10%) and yielded final electrolyte at 30 g/L Cu and below 3 g/L Fe (www.911metallurgist.com). The benefits are measurable: continuous bleed‑and‑makeup maintains high current efficiency; keeping Fe to 3 g/L (via ~10% bleed) preserves nearly 100% efficiency, while without bleed Fe would rise and efficiency could collapse to about 38% (www.researchgate.net; www.911metallurgist.com). The bleed also prevents iron‑induced cathode damage: excess ferric/ferrous cycling can oxidize deposited copper and even cause cathode “hang‑down” failures (www.911metallurgist.com).

This is not optional — it sustains stable operation. From a business standpoint, bleed rate and treatment define losses and costs: if about 10% of Cu goes to bleed, metal yield takes a direct hit. Some SX/EW facilities recycle bleed (or its Cu content) back into leach (if net acid is consumed) to minimize loss, but any discharge must meet local limits (e.g., Indonesian MoE Regulation No. 202/2004 on wastewater quality standard for gold and/or copper ore mining: www.iea.org).

Quantitative summary: typical SX/EW circuits bleed enough volume to hold Fe around 3 g/L with about 30 g/L Cu in the electrolyte (www.911metallurgist.com). At Cu:Fe=100:1, this corresponds to bleeding roughly 10% of SX throughput (www.911metallurgist.com). With crystallization or advanced treatment, Cu loss to bleed can drop to about 4% (www.911metallurgist.com). As Fe in the electrolyte rises, EW current efficiency plummets — from about 99.6% to about 38.2% in one stage — confirming that bleed control, alongside SX purification, directly drives recoveries and product quality (www.researchgate.net).

Performance targets and cited sources

Taken together — precipitation, SX scrubbing, and bleed — the practices above ensure electrolyte entering EW cells is exceptionally pure. The outcome is high cathode quality, high current efficiency, and compliance with environmental standards. For example, an Indonesian copper SX/EW operation would use these controls to meet local effluent rules (e.g., MoE Reg. No. 202/2004: www.iea.org) and to reliably produce cathode copper (99.99% purity) with minimal downtime.

Sources and references used in this article include Izadi et al. (2017) on Fe removal efficiencies (www.researchgate.net), Kaya (2023) on impurity removal techniques (link.springer.com), Yang & Wang (2023) quantifying Fe’s effect on EW efficiency (www.researchgate.net), trade/technical sources on SX/EW design and bleed practice (www.911metallurgist.com; www.911metallurgist.com), EMEW guidance on impurities and halide limits (blog.emew.com; blog.emew.com), and patents detailing electrolyte bleed systems (patents.justia.com; patents.google.com). All figures above — e.g., “~70% Fe removal” by jarosite at pH 2 and 95 °C; “~20%” for hydroxide at the same pH; “capacity ≈42 mg Fe/g” for S957; “Cu:Fe ~100–200:1”; “>6:1” Cu:Fe guidance; “<50 ppm Cl⁻” and “<5 ppm F⁻/Br⁻”; “~30 g/L Cu” and “~3 g/L Fe”; “~10% bleed,” reduced to “~4%” with crystallization; and current efficiency falling from “~99.6% to ~38.2%” — are attributed to those sources at the links provided.