Grind Smart, Leach Fast: Why Slurry Born in the Mill Determines Gold–Copper Recovery

The choice of grinding circuit doesn’t just set throughput; it engineers the particle size distribution (PSD), mineral liberation, and slurry chemistry that leaching inherits. In gold–copper circuits, two‑stage closed ball milling often delivers an “all‑slime” grind (90%+ −74 µm) for rapid cyanide leaching, with first‑stage cyclone overflow at ~55–65% −75 µm and second‑stage at ~90–95% −75 µm (walkermining.com). Industry practice for gold leaching is typically two‑stage closed ball milling to ~90% minus 75 µm (walkermining.com).

But finer isn’t always better. Energy cost rises and ultrafines (“slimes”) can choke mass transfer. Semi‑autogenous grinding (SAG, where partial comminution comes from the ore itself) usually feeds coarse product (P80 on the order of mm–cm) to ball mills for final sizing. Advanced options—high‑pressure grinding rolls (HPGR) or stirred mills—can create more fines and micro‑fractures at lower specific energy; for very fine products (<20 µm), stirred mills can be ~40% more efficient than tumbling mills (metso.com).

Grinding circuit configuration and media chemistry

Closed circuits with cyclones maintain high circulating loads and deliver very fine products; open circuits leave more coarse fragments. Media matters too. A rod+ball circuit using reactive forged steel rods produced slurry with lower redox potential (Eh), lower dissolved oxygen, and higher dissolved iron than a single‑stage ball mill with inert high‑chrome media—conditions that hurt downstream performance (researchgate.net; researchgate.net). In practical terms, less active media in primary mills (ceramic or high‑chrome steel) helps avoid excessive iron dissolution and reducing conditions (researchgate.net; researchgate.net).

Alternate circuits can also rewire leach response. Replacing conventional crushing with HPGR in a copper ore circuit (pyrochlore) produced an ultrafine product (80% <0.8 mm) that increased copper leach extraction from 66% to 74% after 150 days—a +12% recovery gain (researchgate.net). On a gold ore, HPGR at ~5.5 N/mm² created more micro‑cracks and finer particles, boosting gold recovery by ~6.8–10.9% while reducing cyanide consumption by ~5.8–11.1% (researchgate.net).

Particle size distribution and slurry properties

Leaching is a surface reaction, so PSD drives dissolution. Gold dissolution rises as particle size decreases; very fine fractions (<0.020 mm) can exceed 90% gold dissolution even without oxidation pretreatment (mdpi.com; mdpi.com). Yet pushing too fine backfires: slimes (<10 µm) thicken viscosity, slow diffusion, and form hydration layers that block reagent access (mdpi.com). Overgrinding can create gelatinous colloids (notably silica gels) that adsorb gold complexes and entrap dissolved gold, cutting apparent recovery (mdpi.com).

Data point to a balance. One study cites an optimal ~92.5% below 0.074 mm; grinding to −0.074 mm (92.53% passing) plus leaching (28 h, pH 10.5, 4 kg/t KCN) yielded 90.91% gold recovery; adding a mixed “aid‑leaching” reagent for 18 h pushed it to 93.20% with half the cyanide (mdpi.com; mdpi.com; mdpi.com). Typical CIP plants then thicken slurries to ~40–45% solids (wt%) with pH ~10–11 (too thin consumes excess reagents, too thick hampers oxygen diffusion and mixing) (walkermining.com). Maintaining pH in the 10–11 range also prevents HCN formation and keeps cyanide in the OH⁻ form (mdpi.com), and precise pH adjustment avoids both CN loss and metal loss—an area that depends on accurate chemical dosing (dosing pumps).

There is a ceiling: raising pH above ~10.5 began to decrease gold recovery (e.g., at 11.5 pH the rate dropped from ~90% to ~82%) (mdpi.com). In practice, targets often center on ~80–90% passing 75 µm for gold ores—fine enough for >90% dissolution of the fine fraction (mdpi.com; walkermining.com)—while curbing slime generation and power draw.

Chemical additives and slurry conditioning

pH control (CaO, Ca(OH)₂, NaOH). Cyanide must be kept alkaline (pH ≥10) to avoid HCN volatilization; lime is routinely added in grinding/conditioning to stabilize pH ~10–11 (mdpi.com). Han et al. (2024) note pH ~10.5 saved cyanide: at pH 10.5 the gold rate was ~90.9%, whereas at pH 11.5 it dropped to 82.3% (mdpi.com). In practice, one sets pH ≈10.5–11 (depending on cyanide form) during grinding or pre‑leach to safeguard downstream leaching.

Redox/oxidants. Sufficient oxygen is required for cyanidation. Slurries are often aerated or enriched with O₂. Han et al. remark that during grinding, dissolving lime can generate CaO₂ films on gold surfaces which hinder leaching; oxidizing additives (or polishing circuits) can break these films (mdpi.com). Adding small amounts of H₂O₂ during grinding or in early leaching is known to increase dissolution rates without altering bulk slurry chemistry significantly (we did not find a direct reference in above, but it is industry practice).

Dispersion aids. Clay‑rich ores flocculate; sodium silicate or polyphosphates disperse clays and fine silica, preventing coatings and gelatinous slimes and improving access to gold/sulfide surfaces (researchgate.net). Where dispersion is required, plants use dispersants to prevent particle agglomeration (dispersant chemicals), aligning with the paper’s caution that failure to disperse invites silica gels that trap gold complexes (mdpi.com).

Activators/inhibitors. In gold–copper ores, additives can catalyze gold dissolution or suppress deleterious reactions. Examples include trace CuSO₄ (as a catalyst) or low‑dose iodine to increase gold cyanidation rates in some ores (common metallurgical practice, though not cited here). Conversely, dissolved copper consumes cyanide and can foul zinc; copper depressants (e.g., adding lead nitrate or thiourea) or switching to alternative lixiviants (glycine + oxidants for copper) may be used. Recent research shows cyanidation assisted by glycine/oxidant alkaline solutions can enhance gold recovery and lower cyanide demand (sciencedirect.com; sciencedirect.com).

Colloidal control. Aggressive grinding can create colloidal silica that fouls downstream circuits. A silica‑targeting polymer reduced colloidal silica in copper leach solution, boosting copper production by ~20% and cathode quality (>99% Cu spec), delivering a 9% revenue increase (globalminingreview.com). In such contexts, coagulants are used to improve removal of colloids (coagulants). Flocculants are rarely added before leaching as they slow reaction; rather, fine removers or flocculants are used on tailings or at the thickener stage (flocculants).

Reagent consumption. Aid‑leaching (0.6 kg/t) allowed cutting KCN dosage by 50% while raising gold recovery from 91.0% to 93.2% (mdpi.com). HPGR‑improved grind saved 6–11% cyanide in tests (researchgate.net). Each additive must be calibrated: too much silicate at high pH can precipitate calcium silicate on gold; cyanide leach rates plateau beyond ~4 kg/t KCN (mdpi.com).

Aligning comminution and leaching parameters

Since over‑fines generate problematic slimes (mdpi.com), control cyclone cut size and ball charge to achieve liberation without colloids. Downstream, operate leach tanks at ~40–45% solids and pH 10–11 to complement the PSD and maintain cyanide speciation (walkermining.com; mdpi.com). Add pH modifiers and dispersants at grinding discharge or thickener feed; enhancements to carbon adsorption typically occur post‑leach (as noted in conventional CIP practice).

Quantified outcomes from tuned grind–leach operations include: modern CIP circuits achieving 90–95% gold extraction at optimal fineness (mdpi.com; mdpi.com); hybrid comminution (HPGR/stirred mills) adding several percent recovery (7–11% more in lab tests, researchgate.net) and, in copper, ~20% more production after colloid removal (globalminingreview.com); aid‑leaching cutting cyanide by 50% (mdpi.com) or ~10% via HPGR (researchgate.net); and energy gains where HPGR reduced ball‑mill power and allowed ~30% more throughput at Los Colorados Huasco (scielo.org.za), while stirred mills can be ~40% more efficient for ultra‑fine grind (metso.com).

A hypothetical Indonesian gold plant upgrading from artisanal slump‑grind to a controlled ball‑mill circuit with proper pH/dispersion could potentially raise recovery from <60% to >90% (jdmlm.ub.ac.id). Likewise, a silica‑coagulating polymer program saw ~20% higher Cu output and a 9% revenue increase, with >99% Cu cathode spec (globalminingreview.com).

Regulatory controls on cyanide (Indonesia)

Indonesian regulation (PermenLH No. 23/2008) tightly controls cyanide use in mining, requiring tailings treatment to very low cyanide levels (researchgate.net). Optimized grind and additives as above help meet targets (e.g., <0.5 mg/L free CN in effluent) while maximizing extraction: generating fewer slimes and using dispersants reduces cyanide “tie‑up” in colloids, and effective cyanide destruction (sulfur dioxide/air or biological) is employed on tailings—both beyond the grinding scope but relevant to overall compliance strategy.

What the ideal slurry looks like

The ideal product from grinding to leach is well‑liberated but not colloidal, at ~40–45% solids, pH 10–11, and chemically balanced for the chosen reagent. In summary recommendations: target ~90% –¾ mm fineness with closed‑circuit SAG/Ball (or HPGR) grinding, use inert media early to avoid extra Fe, and operate slurry at ~40–45% solids, pH 10–11. Include dispersants if fines‑rich, and consider additives (oxidizers, activators) tailored to ore type. Practice has shown—and sources confirm—that these settings maximize leach kinetics and minimize costly reagents (mdpi.com; researchgate.net; walkermining.com).

Peer‑reviewed studies and industry sources ground the numbers cited here, including 90.9% gold recovery at 92.5% finer −74 µm (mdpi.com) and ~20% higher Cu cathode yield after colloid removal (globalminingreview.com). Where relevant, Indonesian ASGM grind times and regulatory context are noted (jdmlm.ub.ac.id; researchgate.net).