Inside the control room: How SX‑EW plants keep copper on spec, hour by hour

In a market that now demands in‑country refining — Indonesia’s Permen ESDM No. 7/2023 explicitly raises the stakes (peraturan.bpk.go.id) — SX–EW operators are wiring plants with sensors and lab checks to guarantee ≥99.95% Cu (ASTM B115, standards.iteh.ai). The quality plan starts upstream, where pH, [Cu], [Fe], H₂SO₄, trace impurities, temperature, phase ratios, and current density are all watched.

On‑line analyzers (pH probes, flow meters, in‑line copper monitors) and automated controllers for acid dosing keep setpoints tight; tying dosing to a dosing pump with interlocks is standard practice in this context. Laboratory assays — AAS/ICP‑OES and titration — validate those numbers, and all of it feeds into control charts. One study attributed a 5.4% rise in current efficiency, yielding ~74 t of 99.999%‑pure cathode over 1.5 months, to this kind of statistical process‑control discipline (scielo.org.za).

Pregnant leach solution sampling

The pregnant leach solution (PLS) — the acidic liquor emerging from ore heaps — is sampled at least hourly. A typical copper‑oxide heap leach can run around ~2.0 g/L Cu and 2.7 g/L Fe in solution (mdpi.com). The goal is consistent leach efficiency (e.g., 90–95% Cu dissolution) with minimal dissolved Fe/Al. Operators titrate for acid, run ICP/AAS for Cu, Fe, Al, Mn, As, and others, and continuously log pH and redox; keeping pH ~1.5–2.0 matters because pH shifts hit both acid consumption and Cu extraction efficiency.

Mass balance is non‑negotiable — flow × concentration must add up — and any unexplained copper loss triggers root‑cause work (think scaling or a mechanical fault). For solids control, turbidity meters or particle counters ensure the CIL plant (if present) isn’t overloaded with fines, which prevents organic carry‑over. Where installed, an upstream clarifier reinforces that solids barrier ahead of SX.

Targets and alarms are explicit: >90% Cu recovery per pass; acid consumption around 200–400 kg H₂SO₄ per metric tonne of Cu, with PLS pH ~2.0; and any deviation in Cu or Fe by >5% from the daily average prompts corrective action. Trends are watched — a rising Fe or falling Cu often precedes a plant upset.

Solvent extraction stage control

Most circuits run two extraction stages, one to two scrub stages, and two strip stages. In extraction, mixer‑settler pH sits ~1.4–1.6 (for oxime‑ or phosphoric‑based extractants) to load copper and reject iron. In one test at pH 1.6, the Cu distribution ratio exceeded 70 while Fe’s D≈0.12, delivering a Cu/Fe selectivity (β) of ~600 (mdpi.com). Raffinate Cu is driven below <0.5 g/L — and often <0.1 g/L in steady state.

Operators assay organic phase copper (strip‑titration or spectrophotometry) and raffinate Cu (on‑line AAS/XRF). O/A flow ratios and contact times are kept constant; flow meters are checked weekly. Mixer temperatures sit in the 20–30°C band: too high accelerates extractant degradation, too low slows kinetics.

Scrubbing and stripping setpoints

Scrubbing uses mild acid (or dilute PLS) at ~0.5–1.0 M H₂SO₄ to rinse Fe/Al/Mn from the loaded organic before stripping. Washed‑loaded organic should show negligible Fe — typically <0.02 g/L — or the plant adds scrub capacity or imposes a bleed.

Stripping regenerates lean organic and builds copper‑rich electrolyte. Plants maintain strip acid ~150–210 g/L H₂SO₄, with 200 g/L preferred (data.epo.org). Strip liquor Cu lands ~45–60 g/L (researchgate.net, pdfcoffee.com). One pilot reported 44.8 g/L Cu with just 1.4 g/L Fe in strip (researchgate.net).

Because the electrolytic bath originates here, soluble contaminants (Pb, As, Zn) are tracked at strip; analyses reference ASTM B115 limits, which allow Se, Te <2 ppm (standards.iteh.ai). Online interlocks shut acid addition pumps if an acid meter fails, preventing runaway pH.

Organic phase health checks

Solvent quality — extractant plus diluent — is watched continuously. Water pickup is measured by Karl Fischer titration or on‑line density meters; phase entrainment must be <1–2%. If exceeded, operators adjust settler residence time or add a decanter. Free (unused) extractant is tracked via titration; a drop indicates degradation or loss, with a target of >95% active reagent.

Periodic checks extend to biodegradability or solvent drag loss. The organic should remain clear (no suspended solids), and specific gravity should hold within ±2% of the fresh solvent.

Solvent extraction equilibrium shows Cu loading vs pH; operating at pH ~1.6 achieves ~95–99% Cu extraction with minimal Fe uptake (mdpi.com).

Electrowinning electrolyte control

The tankhouse runs on composition discipline: ~40–60 g/L Cu and ~150–210 g/L H₂SO₄ (typically ~180 g/L) are standard, with an industry example of 40 g/L Cu at 180 g/L acid (data.epo.org; pdfcoffee.com). Temperature holds ~40–50°C to lower resistivity (~1.4 Ω·cm, pdfcoffee.com). Chloride sits ~30–80 mg/L for deposit brightness; too much pits, too little dulls (data.epo.org).

Continuous recirculation keeps the bath homogeneous. On‑line sensors watch Cu and specific gravity; significant drift — say Cu <35 g/L or acid <150 g/L — alarms because low acid pushes up cell voltage and promotes impurities. Water quality (deionized) is QC‑checked weekly to prevent scaling or sulfate buildup; many plants generate that makeup via a demineralizer or, for continuous ultrapure service, EDI.

Electrical parameters and monitoring

Current density typically runs 200–300 A/m² (20–30 A/dm²). Many plants use periodic polarity reversal to minimize anode skin; one documented sequence used 15–25 A/ft² forward pulses (≈160–270 A/m²) with short reverse pulses (30–40 A/ft²) and achieved 90%+ cycle efficiency (freepatentsonline.com). PLCs record cell voltages, targeting ~2.0–2.3 V under load; fast rises point to short‑circuiting or anode passivation.

Bath circulation rate and anode–cathode gaps are fixed. A conductivity sensor may be used to detect large particulate in electrolyte (which would increase resistance). Sample filtration before analysis often uses a cartridge filter to remove fines without altering dissolved metals.

Cathode product verification

Finished cathodes must meet purity grades. Namibia’s SX–EW plant has reported Grade‑A copper at 99.999% Cu (scielo.org.za). Final QA includes weighing and assaying sample cathodes. Monthly, cathode samples go out for trace‑metal analysis by ICP‑MS/AES (Se, Te, Pb, Ni, etc.); ASTM B115 specifies Se/Tellurium <2–10 ppm (standards.iteh.ai). Resistivity checks (for conductivity) and morphology (SEM) reveal dendrites or occlusions. Any “off‑spec” cathode (e.g., >0.05% impurity) triggers a review of upstream scrubbing/stripping.

Performance metrics close the loop: >90% current efficiency is expected — a tuned process reported 90–94% — and energy comes in around ~2–3 kWh/kg Cu (freepatentsonline.com). Eliminating voltage “hotspots” in one tankhouse raised efficiency by +5.4% (scielo.org.za), translating into tens of tonnes of additional cathode.

Sampling plan and laboratory methods

A formal sampling plan underpins the whole program. Automated and manual samplers are installed on PLS feed, SX raffinate, loaded/lean organic, strip liquor, and EW electrolytes. Critical streams see hourly grab samples; others are taken shift‑wise. Samples are filtered/acidified on collection if needed (e.g., add HNO₃), with duplicate and blank samples to check consistency. Results are routed into a LIMS and plotted on control charts.

Lab work is standardized: metals (Cu, Fe, and others) by AAS or ICP‑OES following standard methods (e.g., ASTM E203 for Cu); acid content (sulfate) by titration with NaOH (ASTM E200); pH and conductivity by calibrated probes (daily calibrations). Organic‑phase copper is measured by stripping a known volume with acid and assaying that strip, or by phase separation and UV‑vis/colorimetry. Water content in organic goes by Coulometric Karl Fischer with a target <1%.

Quality checks anchor decisions: if a result exceeds specification — for example, Cu concentration off by >3σ of normal — the sample is re‑tested; if confirmed, the process is adjusted immediately (change O/A flow, tweak SX pH, or divert electrolysis cells). Weekly/monthly audits review data, batch yields, and reagent usage records. Major deviations (such as a run of falling current efficiency) trigger root‑cause analysis, from cleaning electrode racks to pulling forward preventative maintenance on pumps/agitators. Where fines management is part of the plan, an upstream clarifier complements the turbidity and particle‑count controls already in place.

Why the discipline pays

Across leaching, SX, and EW, the QC scheme combines continuous monitoring (pH, flows, [Cu]) with periodic laboratory verification. It enforces spec limits at every stage, ensuring consistent high‑grade cathodes that meet ≥99.95% Cu (ASTM B115, standards.iteh.ai) and supporting yield and cost efficiency — as evidenced by the reported 5.4% efficiency gain and ~74 t of 99.999% cathode in 1.5 months (scielo.org.za). For plants that need temporary solids or water polishing during upsets, containerized options and filtration hardware are common parts of the tool kit, complementing the core controls without changing the setpoints that keep copper on spec.