Aldoxime vs Ketoxime: Copper SX Extractant Selection

Last updated: 2026-11-04

The aldoxime vs ketoxime decision comes down to one trade-off: a modified aldoxime system typically transfers 0.4-0.8 g/L of copper per extraction stage against 0.2-0.35 g/L for a ketoxime system (indicative), but strips only at higher acid strength. Choose aldoxime formulations for high-Cu, high-throughput oxide liquors; choose ketoximes for low-Cu, high-impurity bioleach circuits where stripping ease and reagent tolerance dominate.

Key Takeaways

• In solvent extraction for copper, the reagent family sets stage count, strip acid strength, and iron behavior.
• Aldoximes (salicylaldoxime-based) load copper faster and stronger; ketoximes strip easier and cost less per kilogram.
• Modified aldoxime formulations carry modifiers to fix hard stripping and slow phase disengagement.
• At high free acid, unmodified aldoximes can co-load ferric; acid control and a scrub stage manage it.
• Swapping families rarely needs an organic inventory dump; tune strip acid first, then blend gradually.

The Chemistry: How Oxime Extractants Bind Copper

Start from the reaction both families share. A hydroxyoxime is an organic chelating agent that complexes Cu(II) through a phenolic oxygen and an oxime nitrogen, releasing two protons as it does so. Written plainly:

2 HR(org) + CuSO4(aq) → CuR2(org) + H2SO4(aq)

Two ligand molecules wrap one copper ion in a neutral, oil-soluble complex, and the two released protons are the acid release that every downstream number in this article hangs on: extraction consumes acidity, stripping returns it. Loading therefore depends directly on aqueous acidity, and stripping is the same equilibrium reversed with strong sulfuric acid. Every kilogram of copper stripped consumes on the order of 1.5 kg of H2SO4 equivalent (indicative; the exact figure depends on free-acid concentration and how the strip loop is balanced), which is why strip acid strength and inventory dominate the reagent logistics of any copper SX plant.

A salicylaldoxime is an aldehyde-based oxime that forms a markedly stronger copper complex than its ketone counterpart, because the aldehyde-derived carbon next to the oxime nitrogen shifts electron density and tightens the chelate ring. A ketoxime is a ketone-based oxime that loads copper more gently — a lower distribution coefficient that translates directly into easier, lower-acid stripping. A modifier is a polar additive that relaxes the downsides of a strong extractant: it weakens organic-phase self-association just enough to speed stripping and sharpen phase disengagement without killing loading strength.

In service both chemicals dissolve at roughly 7-15 vol% (typical) in a kerosene-type diluent, and the organic circulates between extraction and strip sections as the metal carrier. Practical organic capacity — how much copper it picks up per liter before saturating — follows the 2:1 ligand-to-copper stoichiometry but sits below theoretical because of diluent interactions, modifier effects, and incomplete approach to equilibrium at real residence times. That stoichiometry is the backbone of copper solvent extraction design: it fixes the organic-to-aqueous ratio, the stage count, and the acid balance before a single mixer is sized. The two families do not differ in the reaction; they differ in the equilibrium constant, the rate, and the byproducts they generate as they age — and those three differences drive every selection argument that follows.

Aldoxime vs Ketoxime: How Do the Key Properties Compare?

The aldoxime vs ketoxime trade-off is best read off the extraction isotherm, not the datasheet. An isotherm plots copper in the organic against copper in the aqueous at equilibrium. A modified aldoxime isotherm sits high and steep: the organic loads heavily from even modest aqueous grades. A ketoxime isotherm is lower and flatter. On a stage-construction diagram, the steep aldoxime isotherm lets two extraction stages reach a raffinate that takes three or four stages with a ketoxime — that is the whole capital argument for aldoximes in high-grade duty. The same strength appears in the strip isotherm: copper that loads eagerly at low acidity refuses to leave at moderate acidity, so the strip section must run stronger acid or more stages. Ketoximes invert the bargain — lower loading, easier strip.

Loading speed matters as much as loading depth. Aldoxime kinetics are fast enough that a standard 2-3 minute mixer approaches equilibrium; ketoxime systems need longer residence, more intense mixing, or acceptance of per-stage transfers at the low end of the table. More intense mixing shears droplets finer, which raises organic-in-aqueous entrainment and feeds crud formation — the plant-level hidden cost of a weak extractant. The entrainment and crud interplay runs both ways: fine solids stabilize emulsions into a crud layer that physically holds up reagent, while surfactant-like degradation products of the extractant itself can further stabilize those emulsions.

Degradation is where the families age differently. Oximes hydrolyze and oxidize slowly in service — accelerated by high temperature, chlorides in the feed, and dissolved oxygen — producing aldehyde or ketone fragments and amide-type byproducts that shift phase-disengagement behavior and gradually move the operating isotherm. This is why active oxime concentration must be assayed on a schedule, never assumed from the original mix ticket.

Modifier and diluent logic follows directly. Because unmodified salicylaldoxime strips poorly and disengages slowly, aldoxime products sold for copper plants carry modifiers — alcohol, ester, or phenolic types — tuned to relax the strip end of the isotherm and speed disengagement. Ketoximes typically run unmodified in simpler diluents, at some cost in kinetics. The selection consequence is simple: an aldoxime choice commits you to a formulated product and tighter strip-acid control; a ketoxime choice commits you to more mixer volume.

O/A ratio consequences follow from the same isotherms. With a strong extractant, extraction runs near O/A 1:1 to 1.5:1 (typical) and still finishes the pass; with a ketoxime, moving the same copper per stage at O/A near 1:1 is often not possible, so plants raise organic flow, add stages, or accept higher raffinate copper. Every number below is indicative and must be confirmed on your own pregnant leach solution (PLS) by supplier isotherm test work — the same strength-versus-strippability balance that decides other systems, as covered in D2EHPA vs Cyanex 272.

Aldoxime vs Ketoxime by Feed Scenario: Which Family Wins Where?

The aldoxime vs ketoxime question resolves differently by feed, and pretending one family wins everywhere is how reagent budgets get wasted. Score each scenario on three properties from the table above: capacity, strip acid demand, and impurity behavior.

High-Cu, high-acid PLS from oxide leach: modified aldoxime wins. At 4-6 g/L Cu feed, a ketoxime’s flat isotherm forces three to four extraction stages and a large organic inventory to move the same metal, while a modified aldoxime routinely finishes in two. Warmer oxide-leach PLS suits the fast aldoxime kinetics, and the higher strip acid demand is trivial for a plant already sending strong acid to strip. This is the standard duty across most copper industry applications served by SX-EW today, and it is why aldoxime formulations dominate new high-throughput designs.

Low-Cu, high-impurity chalcocite or bioleach liquor: ketoxime wins. With 1-2 g/L Cu feed, the copper movement demanded per stage is small, so aldoxime capacity is wasted money. Ketoximes strip at milder acid, tolerate feed excursions without catastrophic isotherm shifts, and cost less per kilogram. Slower kinetics barely matter when the ΔCu per stage is a few tenths of a gram per liter. Verdict: unless throughput forces otherwise, stay with the ketoxime.

Mixed circuits and reagent swaps. For fluctuating or blended feeds, a mid-strength modified aldoxime — or deliberately blended reagents — gives the most forgiving isotherm. Can you swap families without dumping the organic inventory? Usually yes: the families are chemically compatible hydroxyoximes in similar diluents, so most circuits convert by blending the new reagent in 10-20% increments over several weeks. One warning only a plant metallurgist tends to learn the hard way: during a swap trial, watch strip acid inventory before you watch loading. If the spent electrolyte is too weak for the stronger incoming aldoxime, loaded copper returns to the extraction section and the trial “fails” for reasons that have nothing to do with extraction chemistry. Verify the strip loop can hold the higher acid strength — tank capacity, acid plant allocation, and g/L control — before the first drum of aldoxime is poured.

What Goes Wrong in the Plant, and Why Do Swaps Fail?

Most aldoxime vs ketoxime disappointments trace to a short list of recurring failure modes. Each item below follows the same discipline: phenomenon, then cause, then fix.

1. Strip collapse after a family swap. Strip liquor copper falls, copper circulates between sections, and raffinate copper rises within days. Cause: strip acid strength set for the old ketoxime cannot strip the stronger aldoxime complex. Fix: raise strip acid to the supplier-confirmed strength for the new reagent and confirm spent-electrolyte tank inventory and acid supply before the trial, not after.
2. An isotherm “change” that is really concentration drift. Per-stage transfer falls over months with no feed change, and the team blames the ore. Cause: diluent evaporation, selective solubility losses, and degradation have quietly lowered the active oxime vol%. Fix: assay active oxime concentration by the supplier’s titration method on settled organic from each section, top up to a fixed vol%, and trend it against per-stage transfer — the “isotherm” often snaps back within a week.
3. Iron loading at high acid. Iron creeps into the electrolyte and cathode quality slips. Cause: unmodified aldoxime co-loads ferric at high free acidity. Fix: move to a modified formulation, hold extraction-section acid in the designed band, and add or retune a scrub stage; verify with an Fe isotherm on the actual PLS.
4. Phase disengagement decay and crud growth. Disengagement times lengthen and a persistent crud band forms at the interface. Cause: degradation byproducts plus entrained fine solids stabilize the emulsion and hold up organic. Fix: treat the organic (clay or centrifuge), cut solids at the source, and review the degradation path in extractant degradation; the disengagement test sequence in phase disengagement isolates chemistry from solids.
5. Cold-weather kinetics slump in ketoxime plants. Every winter, per-stage transfer drops and operators add residence time that does not exist. Cause: ketoxime kinetics slow markedly below about 20 °C. Fix: raise mixer residence or intensity, increase oxime vol%, or — if winters are long — reconsider the family choice entirely.
6. Organic entrainment reaching the tankhouse. Organic films appear on electrolyte and cathode quality suffers. Cause: over-mixed, finely sheared droplets and weak coalescence, usually aggravated by the mixing intensity that a slow ketoxime invited. Fix: coalescing filters or froth flotation on raffinate and electrolyte bleeds, mix-intensity control, and honest measurement of entrainment before blaming the reagent.

None of these modes argues against either family; they argue for testing the reagent in the plant’s own liquor and instrumenting the trial so chemistry is not blamed for a utilities problem.

Cost per Kilogram of Copper Moved

The honest comparator is not price per drum but reagent cost per kilogram of copper moved: cost per kg Cu = consumption (kg extractant per t Cu) × price ($ per kg) ÷ 1,000. Consumption must include everything — entrainment losses, aqueous solubility, degradation replacement — because those, not neat reagent price, usually dominate. All figures below are indicative ranges, to be confirmed by supplier test work and an on-site organic inventory audit.

Read the table with its assumptions stated: prices exclude duty and freight swings; consumption assumes a reasonably tight plant with entrainment control; diluent losses are excluded but are often the largest single organic loss stream and are strongly site-specific. Note what does not appear as a cost: strip acid. The roughly 1.5 kg H2SO4 per kg Cu consumed in stripping is regenerated in the extraction section, so it is a circulation and control problem, not a net consumption — until a swap to a stronger extractant forces higher acid strength that the acid plant or storage cannot comfortably hold.

On pure reagent cost per kilogram moved, the ketoxime is cheaper. The aldoxime’s case is made elsewhere: each avoided extraction stage carries mixer-settler capex, diluent inventory, and agitation power, so at high feed grade the stronger reagent usually wins the total-cost comparison even while losing the price-per-kilogram one. At low feed grade, where stages are not the constraint, the ketoxime’s cheaper chemistry generally wins. Cost per kilogram of copper moved is therefore a feed-grade decision before it is a price-list decision.

Two further cost lines belong in any honest comparison, even though neither fits the formula above. Crud treatment — clay treatment batches, centrifuge time, and the organic that a crud layer traps and discards — can add materially to annual consumption in plants with dirty feeds, and it punishes whichever family is run with more aggressive mixing to compensate for slow kinetics. Second, swap costs are real but small: converting between families costs a few weeks of blended operation and one or two extra assay rounds, not an organic dump, which is precisely why family choice should be revisited whenever feed grade shifts materially rather than defended by sunk inventory.

Worked Example: Sizing a Swap on a Bioleach Circuit

Consider an anonymized bioleach circuit we reviewed. PLS flows 600 m³/h at 1.6 g/L Cu with 8 g/L free acid at 35 °C. Copper fed is 600 × 1.6 = 960 kg/h; the target raffinate is 0.20 g/L, so the plant moves 840 kg/h of copper — 6,720 t/y over 8,000 operating hours.

The incumbent is an unmodified ketoxime at 11 vol%. Plant tag data show a net transfer of 0.35 g/L per extraction stage (within the indicative 0.2-0.35 band for this feed grade), so removing 1.40 g/L takes exactly 4.0 stages — and the plant has 4, which is why it works but has no margin.

A vendor proposed a modified aldoxime. At the same feed, indicative per-stage transfer is 0.55 g/L, so 1.40 ÷ 0.55 = 2.5, rounded to 3 extraction stages: one mixer-settler train freed. But the strip side demands the attention: the existing spent electrolyte at 150 g/L H2SO4 is below the 175 g/L (indicative) the aldoxime needs for clean strip. The 2,000 m³ spent-electrolyte tank would hold 300 t of acid at 150 g/L versus 350 t at 175 g/L — an inventory and acid-supply question to settle before any trial.

The money: ketoxime consumption of 3.2 kg per t Cu at $7.5 per kg gives $24 per t Cu, or 6,720 × 24 = $161,280 per year. Aldoxime consumption of 4.0 kg per t Cu at $9.50 gives $38 per t Cu, or $255,360 per year — a $94,080 per year reagent penalty. Against one idled mixer-settler and its diluent inventory, the trade was not compelling at this feed grade, and the recommendation was to keep the ketoxime and bank the margin. The one caveat recorded for the future: if the leach feed grade rises above roughly 2.5 g/L Cu, the four-stage ketoxime train runs out of margin first, and the same arithmetic should be rerun before capacity is pushed. The numbers, not the vendor brochure, made the call.

FAQ

Can I blend aldoxime and ketoxime extractants in one circuit?

Yes. Both are hydroxyoximes carried in similar diluents and are generally compatible, so most circuits convert by blending the new reagent in increments over several weeks instead of dumping inventory. A blend loads and strips between the two parent isotherms. Confirm compatibility and the required strip acid strength with the supplier, and track active oxime concentration by assay throughout the transition.

Which family strips with less acid?

The ketoxime. Indicative strip acid strengths are about 140-160 g/L H2SO4 for ketoxime systems versus 170-200 g/L for modified aldoxime systems, with exact values set by the specific formulation and confirmed by strip isotherm test work. This is the single most common reason ketoximes survive in older, acid-constrained plants even when their extraction capacity is no longer ideal.

Do aldoxime extractants really load iron?

They can, at high free acidity, if the formulation is unmodified. Modified aldoxime formulations and a controlled extraction-section acid band reduce the effect, and a scrub stage removes what still loads. Because the consequence is iron circulating to the tankhouse, any aldoxime selection should include an Fe isotherm run on your own PLS, not just a copper isotherm.

How is active oxime concentration measured and maintained?

Sample settled organic from the extraction and strip sections and assay by the supplier’s titration or wet-chemical method — each family has its own procedure. Run monthly at minimum, and always after major makeup events. Maintain a fixed vol% by scheduled additions rather than reacting to symptoms; drifting concentration quietly moves the isotherm before anything else changes in the plant.

What O/A ratio is typical in copper SX?

Around 1:1 in extraction is the common starting point (typical), moving toward 1.5:1 with weaker extractants or higher feed grades, and often organic-continuous in strip. The isotherm decides: construct the stages at the real O/A rather than assuming 1:1 works, because a ketoxime at high feed grade frequently cannot move the required copper per stage at O/A near unity.

How long does oxime extractant last in service?

Site-specific and best expressed as consumption rather than age: indicative total makeup is 2-6 kg per t of copper moved including entrainment, solubility, and degradation losses. Degradation accelerates with temperature, chlorides, and dissolved oxygen. Trend active oxime assay and organic inventory over time; a rising consumption rate is an early warning of entrainment or crud losses, not necessarily chemical breakdown.

For a new high-Cu oxide plant, which family should the aldoxime vs ketoxime analysis favor?

A modified aldoxime, decisively. High feed grade makes organic capacity and kinetics the binding constraints: two extraction stages versus three or four saves mixer-settler capex, diluent inventory, and power that dwarf the higher reagent price per kilogram. Confirm with loading and strip isotherms on your PLS, and design the strip acid system for the stronger reagent from day one.

Conclusion

The aldoxime vs ketoxime choice is not a chemistry debate; it is a feed-grade decision with a strip-acid rider. Aldoximes load more copper, faster, per stage, and pay for themselves wherever stage count and organic inventory bind — high-Cu oxide liquor above roughly 3 g/L Cu. Ketoximes strip easier, tolerate high-impurity low-grade bioleach liquors, and cost less per kilogram moved. Between those poles, the deciding evidence is an isotherm on your own PLS and an honest strip-loop audit, because the most expensive extractant is the one whose strip section was never resized. If you want loading and strip isotherms run on your actual liquor, with a stage-count and reagent-cost comparison you can defend, request a quote for a copper SX reagent selection program — and put the family question to rest with data.

References

• Davenport, W.G., King, M., Schlesinger, M. and Biswas, A.K., Extractive Metallurgy of Copper, Elsevier.
• Ritcey, G.M., Solvent Extraction: Principles and Applications to Process Metallurgy, revised edition, G.M. Ritcey & Associates.
• Szymanowski, J., Hydroxyoximes and Copper Hydrometallurgy, CRC Press.
• Kordosky, G.A., “Copper Recovery Using Leach/Solvent Extraction/Electrowinning Technology,” JOM.
• Flett, D.S., “Solvent Extraction in Copper Hydrometallurgy: A Review,” Hydrometallurgy.
• Habashi, F., A Textbook of Hydrometallurgy, Métallurgie Extractive Québec.
• International Solvent Extraction Conference (ISEC) Proceedings, Canadian Institute of Mining, Metallurgy and Petroleum.

Changelog

• 2026-11-04: Initial publication.