When Ore Fights Back: Carbon steals gold, carbonates swallow acid — and the chemistry that wins it back

Leach plants live and die by what’s in the rock. For gold, indigenous carbon can “preg-rob” the circuit — adsorbing dissolved Au–CN complexes (gold–cyanide species) before the metal reaches carbon or resin. For copper, acid-hungry gangue minerals, especially carbonates and reactive silicates, soak up H₂SO₄ long before it can attack copper minerals. Both phenomena are mineralogical, both are costly, and both respond to chemical pre-treatment.

The literature is unambiguous: preg-robbing alone can wipe out more than half of the gold that cyanide puts into solution, depending on the type and maturity of the carbonaceous matter ([gmj.umat.edu.gh](https://gmj.umat.edu.gh/ojs/index.php/gmj/article/view/300#:~:text=Depending%20on%20the%20type%20and,gold%20processing%20and%20estimates%20the)). And even “modest” carbonate in a copper ore can demand tens of kilograms of H₂SO₄ per tonne before any copper dissolves ([www.mdpi.com](https://www.mdpi.com/2075-163X/8/2/47#:~:text=Calcite%20is%20a%20well,thus%20represent%20a%20relatively%20simple)).

Gold leaching: carbonaceous preg-robbing

Preg-robbing (ore carbon adsorbing leached gold) is driven by the maturity of indigenous carbon (graphitic matter, humic substances). Laboratory work shows losses from a few percent to more than 50% of leached gold as carbon becomes more “activated” ([gmj.umat.edu.gh](https://gmj.umat.edu.gh/ojs/index.php/gmj/article/view/300#:~:text=Depending%20on%20the%20type%20and,gold%20processing%20and%20estimates%20the)). Amankwah et al. (2017) measured gold adsorption capacities over 24 hours of ~30–65 g Au/t carbon for wood chips (low maturity), 320–370 for charcoal, 410–420 for barren carbon, and 580–650 for highly activated carbon — a >10× step-up from wood chips to activated carbon ([gmj.umat.edu.gh](https://gmj.umat.edu.gh/ojs/index.php/gmj/article/view/300#:~:text=of%20gold%20per%20tonne%20of,Of%20all%20the)) ([gmj.umat.edu.gh](https://gmj.umat.edu.gh/ojs/index.php/gmj/article/view/300#:~:text=results%20showed%20an%20increasing%20trend,Of%20all%20the)).

On the ground, “black shale” or lignite in ore can lock tens of kilograms of Au per tonne of carbon; unrecovered, this drives up reagent use and shrinks payout. Plants quantify “preg-robbing potential” as the percent of a standard Au–CN solution adsorbed by the ore; values over ~20% are considered “active” and typically trigger pre-treatment ([www.911metallurgist.com](https://www.911metallurgist.com/blog/bioleaching-preg-robbing-gold-ore/#:~:text=200%20Tyler%20mesh%20%2875%20%C2%B5m%29,%E2%80%9D)). In one refractory ore with ~6.95% C, cyanide extraction rose from ~28.7% to 75–80% Au only after bio-oxidation removed or chemically inactivated the carbon ([www.mdpi.com](https://www.mdpi.com/2075-4701/10/9/1216#:~:text=and%20L,Moreover%2C%20passivation)).

CIL, RIL, and adsorbent selection

CIL (carbon-in-leach) loads dissolved gold onto carbon; RIL (resin-in-leach) uses ion-exchange resin. The choice matters when preg-robbing is present. Ocampo-López et al. (2024) showed that protonated activated carbon (PAcC) in a CIL test delivered ~90%+ Au recovery, with only 0.13% Au left in solution — a marker of high scavenging power ([www.mdpi.com](https://www.mdpi.com/2075-4701/14/12/1465/xml#:~:text=This%20study%20evaluates%20the%20preg,4%E2%88%921%7D%20fractional)) ([www.mdpi.com](https://www.mdpi.com/2075-4701/14/12/1465/xml#:~:text=9,demonstrate%20the%20superiority%20of%20PAcC)). Where operators specify adsorbents, sourcing often includes activated carbon suitable for CIL circuits (see /products/activated-carbon) or ion-exchange media matched to RIL service (see /products/ion-exchange-resin).

Chemical pre-treatment for gold ores

The playbook starts with flotation and reagent “blinding” — coating or removing reactive carbon before cyanidation. Valdivieso-Bermeo et al. (2020) floated a Colombian carbonaceous ore using a maltodextrin polymer to depress organic carbon, then washed with a benzalkonium chloride surfactant before CIL. Preg-robbing fell from 39.5% to 3.96%, and gold extraction reached 87.3% (near zero without treatment) ([www.researchgate.net](https://www.researchgate.net/publication/346448601_Treatment_for_gold_ores_with_high_content_of_carbonaceous_matter#:~:text=process%20to%20effectively%20recover%20Au,Au%20was%20achieved%20during%20cyanidation)). Mubarok and Irianto (2016) simulated 2 wt% active-carbon in ore and found kerosene/diesel blinding alone raised Au recovery by ~25% (to ~25.4%); combining blinding with resin-in-leach (Lewatit MP-800) delivered ~99.5% Au recovery ([www.researchgate.net](https://www.researchgate.net/publication/307524272_Improving_Gold_Recovery_from_Artificial_Preg-Robbing_Ore_by_Pre-treatment_using_Blinding_Agent_and_Resin-in-Leach#:~:text=preg,activated%20carbon%20and%20without%20pretreatment)).

Oxidation is another lever. Pressure oxidation (PO) or high-temperature roasting attacks sulfides and modifies organics. A Barrick patent notes PO at ~200°C for 20–100 minutes can partially deactivate ore carbon but is often insufficient alone, recommending additional blanking agents such as kerosene or surfactants ([www.freepatentsonline.com](https://www.freepatentsonline.com/5536297.html#:~:text=recovery,leach%20has)). Bio-oxidation (Acidithobacillus, Leptospirillum) raised thiourea-leachable Au from 28.7% to 75–80% in a double-refractory ore; surface analyses showed –OH/COOH groups introduced and organic bonds degraded, reducing carbon’s gold affinity ([www.mdpi.com](https://www.mdpi.com/2075-4701/10/9/1216#:~:text=and%20L,Moreover%2C%20passivation)).

Alkaline conditioning has emerged as well. HiTeCC applies a high-temperature NaOH leach to carbonaceous ore or CIL residue; early tests suggest part of the gold preg-robbed onto ore carbon can be desorbed under hot caustic and re-adsorbed on fresh carbon — treating preg-robbing as partially reversible ([repository.up.ac.za](https://repository.up.ac.za/handle/2263/78309#:~:text=Metso%20Outotec%20South%20Africa%20developed,The)). Alternative lixiviants are in play: Barrick’s process pairs pressure oxidation with ammoniacal thiosulfate at neutral pH to avoid Au loss to preg-robbing ([www.freepatentsonline.com](https://www.freepatentsonline.com/5536297.html#:~:text=8,ore%20slurry%20with%20the%20lixiviant)). Where the program relies on reagents, operators lean into specialized mining chemicals for these steps (see /products/chemical).

Copper leaching: acid-consuming gangue

Copper ores frequently carry acid consumers. Classic carbonates — calcite (CaCO₃) and dolomite (CaMg(CO₃)₂) — dissolve readily in sulfuric acid, neutralizing protons: CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O. As a rule of thumb, each 1% CaCO₃ requires roughly 10 kg H₂SO₄ per tonne to neutralize, so 5–10% calcite can consume 50–100 kg/t before any copper dissolves ([www.mdpi.com](https://www.mdpi.com/2075-163X/8/2/47#:~:text=Calcite%20is%20a%20well,thus%20represent%20a%20relatively%20simple)). Dolomite and magnesite consume similar amounts per formula unit. Silicates (clays, feldspars, mica, chlorite, amphiboles) react more slowly but still bind acid over time, releasing Mg/Fe/Al and forming secondary phases; clays can gelatinize and form silica–Al gels that immobilize acid ([www.mdpi.com](https://www.mdpi.com/2075-163X/8/2/47#:~:text=The%20silicate%20structures%20may%20be,formation%20of%20silica%20gel%2C%20owing)) ([www.mdpi.com](https://www.mdpi.com/2075-163X/8/2/47#:~:text=plagioclase,15%2C19)).

The acid bill can be brutal. Azmayandeh et al. (2017) reported 93–107 kg H₂SO₄/t in short bottle tests at ~pH 1 to reach ~60% Cu extraction; longer column tests still ran 18–54 kg/t under favorable conditions ([www.researchgate.net](https://www.researchgate.net/publication/319276359_Investigation_of_Affecting_Parameters_on_Heap_Leaching_Performance_and_Reducing_Acid_Consumption_of_Low_Grade_Oxide-Sulfide_Copper_Ore#:~:text=60.2%20and%2060.1,After%20diagnostic)) ([www.researchgate.net](https://www.researchgate.net/publication/319276359_Investigation_of_Affecting_Parameters_on_Heap_Leaching_Performance_and_Reducing_Acid_Consumption_of_Low_Grade_Oxide-Sulfide_Copper_Ore#:~:text=15.2,Copper%20and%20iron)). Industry observation concurs: significant calcite or silicate slows leaching via gypsum precipitation or silica gel formation and sharply raises acid demand ([www.researchgate.net](https://www.researchgate.net/publication/353908672_Understanding_gangue_acid_consumption_in_copper_sulfide_heap_leaching_Predicting_the_impact_of_carbonates_silicates_and_secondary_precipitates#:~:text=,)).

Chemical pre-treatment for acid consumers

Segregation and sequencing help. A Barrick/SRN patent (2009) describes floating an “acid consumer” fraction (carbonates, humic carbon) into a tailings stream and then using that tailings later to neutralize acid generated during pressure oxidation (PO); contacting hot column off-gas with the tailings leverages heat, and mixing into oxidized slurry buffers acidity. Tests halved the lime required in neutralization ([www.patentsencyclopedia.com](https://www.patentsencyclopedia.com/app/20090071296#:~:text=1,d)) ([www.patentsencyclopedia.com](https://www.patentsencyclopedia.com/app/20090071296#:~:text=contacting%20the%20off%20gas%20with,from%20the%20pregnant%20leach%20solution)) ([www.patentsencyclopedia.com](https://www.patentsencyclopedia.com/app/20090071296#:~:text=use%20of%20heat%20exchanged%20tails,EXPERIMENTAL)).

Without flotation, plants often apply a dilute sulfuric acid pre-leach to neutralize carbonates so the autoclave feed remains acidic; one patent notes this explicitly for calcite-bearing feeds ([www.freepatentsonline.com](https://www.freepatentsonline.com/5536297.html#:~:text=Pressure%20oxidation%20is%20typically%20performed,4%2C552%2C589)). After autoclaving, if sulfide oxidation supplies acid, operators neutralize with lime or limestone to precipitate CaSO₄ rather than carry free acid downstream ([www.freepatentsonline.com](https://www.freepatentsonline.com/5536297.html#:~:text=heavy%20metals,followed%20by%20recovery%20of%20gold)). In heaps, agglomeration chemistry can be tuned: although not aimed at gangue, the CuproChlor process (CaCl₂ binder) creates a chloride medium that aggressively leaches chalcopyrite, partially decoupling acid supply from copper dissolution; adding sulfate-liberating salts (e.g., CuSO₄ or Fe₂(SO₄)₃) during agglomeration can “recycle” acid within the heap, offsetting carbonate neutralization losses. The principle is straightforward: ensure added acid does useful work dissolving Cu, not just neutralizing gangue.

Economically, acid shows diminishing returns if gangue dominates: in the same Azmayandeh et al. study, raising irrigant from 7 to 11 g/L H₂SO₄ lifted Cu recovery only from ~25.5% to ~58.9%, while acid use jumped from 18 to 54 kg/t ([www.researchgate.net](https://www.researchgate.net/publication/319276359_Investigation_of_Affecting_Parameters_on_Heap_Leaching_Performance_and_Reducing_Acid_Consumption_of_Low_Grade_Oxide-Sulfide_Copper_Ore#:~:text=60.2%20and%2060.1,After%20diagnostic)) ([www.researchgate.net](https://www.researchgate.net/publication/319276359_Investigation_of_Affecting_Parameters_on_Heap_Leaching_Performance_and_Reducing_Acid_Consumption_of_Low_Grade_Oxide-Sulfide_Copper_Ore#:~:text=15.2,Copper%20and%20iron)). That is why “correct pretreatment” in agglomeration and curing is emphasized to shorten leach cycles and reduce acid consumption ([www.mdpi.com](https://www.mdpi.com/2075-4701/11/7/1067#:~:text=a%20correct%20pretreatment%20process%20include,of%20pretreatment%20in%20agglomeration%20and)).

Mineralogy-first design and outcomes

Characterization drives flowsheet choice. XRD and QEMSCAN to quantify carbonate, clay, and organics inform whether to pre-float, bio-oxidize, pressure-oxidize, blind with surfactants or oils, or switch lixiviants ([www.mdpi.com](https://www.mdpi.com/2075-163X/8/2/47#:~:text=In%20this%20respect%2C%20it%20is,proportions%20of%20such%20carbonates%20are)). Results from the literature are stark: flotation plus surfactant rinsing dropped preg-robbing from ~40% to <4% and enabled 87.3% extraction ([www.researchgate.net](https://www.researchgate.net/publication/346448601_Treatment_for_gold_ores_with_high_content_of_carbonaceous_matter#:~:text=process%20to%20effectively%20recover%20Au,Au%20was%20achieved%20during%20cyanidation)); using PAcC left only 0.13% Au in solution (~90%+ recovery in the CIL test) ([www.mdpi.com](https://www.mdpi.com/2075-4701/14/12/1465/xml#:~:text=This%20study%20evaluates%20the%20preg,4%E2%88%921%7D%20fractional)) ([www.mdpi.com](https://www.mdpi.com/2075-4701/14/12/1465/xml#:~:text=9,demonstrate%20the%20superiority%20of%20PAcC)); combined blinding and resin-in-leach achieved ~99.5% recovery versus virtually none without treatment ([www.researchgate.net](https://www.researchgate.net/publication/307524272_Improving_Gold_Recovery_from_Artificial_Preg-Robbing_Ore_by_Pre-treatment_using_Blinding_Agent_and_Resin-in-Leach#:~:text=preg,activated%20carbon%20and%20without%20pretreatment)). On the copper side, segregating acid consumers and re-using them to neutralize autoclave discharge halved lime in tests ([www.patentsencyclopedia.com](https://www.patentsencyclopedia.com/app/20090071296#:~:text=use%20of%20heat%20exchanged%20tails,EXPERIMENTAL)).

Source literature and patents

Technical sources include Amankwah et al. (2017) on carbonaceous Au ores ([gmj.umat.edu.gh](https://gmj.umat.edu.gh/ojs/index.php/gmj/article/view/300#:~:text=Depending%20on%20the%20type%20and,gold%20processing%20and%20estimates%20the)) ([gmj.umat.edu.gh](https://gmj.umat.edu.gh/ojs/index.php/gmj/article/view/300#:~:text=of%20gold%20per%20tonne%20of,Of%20all%20the)), Mubarok & Irianto (2016) on blinding and resin-in-leach ([www.researchgate.net](https://www.researchgate.net/publication/307524272_Improving_Gold_Recovery_from_Artificial_Preg-Robbing_Ore_by_Pre-treatment_using_Blinding_Agent_and_Resin-in-Leach#:~:text=preg,activated%20carbon%20and%20without%20pretreatment)), Valdivieso-Bermeo et al. (2020) on surfactant-assisted flotation ([www.researchgate.net](https://www.researchgate.net/publication/346448601_Treatment_for_gold_ores_with_high_content_of_carbonaceous_matter#:~:text=process%20to%20effectively%20recover%20Au,Au%20was%20achieved%20during%20cyanidation)), Tang et al. (2020) on bio-oxidation ([www.mdpi.com](https://www.mdpi.com/2075-4701/10/9/1216#:~:text=and%20L,Moreover%2C%20passivation)), and Ocampo-López et al. (2024) on PAcC in CIL ([www.mdpi.com](https://www.mdpi.com/2075-4701/14/12/1465/xml#:~:text=This%20study%20evaluates%20the%20preg,4%E2%88%921%7D%20fractional)) ([www.mdpi.com](https://www.mdpi.com/2075-4701/14/12/1465/xml#:~:text=9,demonstrate%20the%20superiority%20of%20PAcC)). Copper leach data and mechanisms draw on Chetty (2018) ([www.mdpi.com](https://www.mdpi.com/2075-163X/8/2/47#:~:text=Calcite%20is%20a%20well,thus%20represent%20a%20relatively%20simple)) ([www.mdpi.com](https://www.mdpi.com/2075-163X/8/2/47#:~:text=The%20silicate%20structures%20may%20be,formation%20of%20silica%20gel%2C%20owing)), Azmayandeh et al. (2017) column tests ([www.researchgate.net](https://www.researchgate.net/publication/319276359_Investigation_of_Affecting_Parameters_on_Heap_Leaching_Performance_and_Reducing_Acid_Consumption_of_Low_Grade_Oxide-Sulfide_Copper_Ore#:~:text=60.2%20and%2060.1,After%20diagnostic)) ([www.researchgate.net](https://www.researchgate.net/publication/319276359_Investigation_of_Affecting_Parameters_on_Heap_Leaching_Performance_and_Reducing_Acid_Consumption_of_Low_Grade_Oxide-Sulfide_Copper_Ore#:~:text=15.2,Copper%20and%20iron)), and reviews of agglomeration/pretreatment benefits ([www.mdpi.com](https://www.mdpi.com/2075-4701/11/7/1067#:~:text=a%20correct%20pretreatment%20process%20include,of%20pretreatment%20in%20agglomeration%20and)). Patents (Barrick Gold, 1996/2009) document acid pre-treatment, PO–thiosulfate gold recovery, and heated tails neutralization ([www.freepatentsonline.com](https://www.freepatentsonline.com/5536297.html#:~:text=Pressure%20oxidation%20is%20typically%20performed,4%2C552%2C589)) ([www.freepatentsonline.com](https://www.freepatentsonline.com/5536297.html#:~:text=oxidation%20of%20the%20sulfides%20in,4%2C552%2C589)) ([www.freepatentsonline.com](https://www.freepatentsonline.com/5536297.html#:~:text=recovery,leach%20has)) ([www.patentsencyclopedia.com](https://www.patentsencyclopedia.com/app/20090071296#:~:text=1,d)) ([www.patentsencyclopedia.com](https://www.patentsencyclopedia.com/app/20090071296#:~:text=contacting%20the%20off%20gas%20with,from%20the%20pregnant%20leach%20solution)) ([www.patentsencyclopedia.com](https://www.patentsencyclopedia.com/app/20090071296#:~:text=use%20of%20heat%20exchanged%20tails,EXPERIMENTAL)).