Inside the kiln: how gold mines resurrect activated carbon — and protect recovery

Every tonne of ore can quietly erode a mine’s margins. One study pegged activated carbon losses in gold CIP at about 64 g per tonne of ore processed (0.064 kg/t), with roughly 40% of those losses occurring during thermal regeneration and 60% during agitation and handling (www.researchgate.net). For a 2 Mt/y operation, 64 g/t implies ~128 t of makeup activated carbon per year; at ~$2,000–3,000/t, that is ~$0.26–0.38 million annually (www.researchgate.net).

In CIP — an adsorption process in which gold-bearing pulps contact carbon — activated carbon (often coconut-shell granular activated carbon, “GAC,” with ~1,000 m²/g surface area) circulates through agitated leach tanks (10–15 g/L pulp density) to load dissolved Au before elution and reuse (studylib.net). Many plants source or specify activated carbon that tolerates repeated cleaning and reactivation without excessive fines.

The stakes are high: without regeneration, inorganic scales — mainly calcium carbonate from lime dosing — and organics clog pores and slash capacity. Over many cycles, a spent carbon’s adsorptive capacity can drop; acid washing and thermal regeneration aim to remove scale and burn off foulants so carbon returns close to virgin performance (studylib.net).

CIP carbon loading and fouling

Carbon is loaded in tanks, eluted of Au, and “normalized” before reuse; maintaining adsorption capacity is the key performance driver (studylib.net). The main inorganic foulants are calcium carbonate and other metal precipitates, including Mg and Na salts and occasionally Cu/Fe hydroxides; these physically block pores. Organic foulants (tars, oils, humics) are generally minor in gold CIP and are much lower than in water-treatment carbon service (studylib.net).

Acid washing: dissolving the scale

Most mines wash carbon with dilute mineral acid to dissolve scale before or after elution. Hydrochloric acid (HCl) is standard at 2–5% w/w; HCl converts CaCO₃ scale to soluble CaCl₂ (studylib.net). Typical plant practice is 3–5% HCl for ~1–2 h in a dedicated acid-wash column or tank (walkermining.com), with one study showing a 3% HCl soak (1 bed-volume, 1 h contact) removed >95% of calcium from carbon loaded <2% Ca (www.researchgate.net).

Acid-wash outcomes are material: without it, carbon beds can become “cemented” by CaCO₃ scale; with it, plants report 15–25% activity gains relative to unwashed poisoned carbon when aggressive conditions are used (patents.google.com) (patents.google.com). Process engineers often treat the acid program like any other reagent system — specified, metered, and monitored with the same discipline as a chemical supply in the plant.

Specialty acid formulations (HF₂⁻)

In difficult cases with silicate or fluorocarbonate scale, operators add ammonium bifluoride (HF₂⁻) to the HCl wash. One patented recipe uses ~5–12% HCl with 1–2% NH₄HF₂ (roughly a 5:1 weight ratio) (patents.google.com). In tests, 6% HCl + 1% NH₄HF₂ achieved ~20–25% more adsorptivity versus untreated spent carbon, whereas 5% HCl alone gave only 5–10% improvement (patents.google.com). Iron, nickel, and zinc cyanides can also be impacted in acid, with metals forming soluble chlorides (studylib.net).

Caustic rinse and safety controls

After acid washing, the spent liquor — rich in CaCl₂ and metals — is rinsed out. A dilute caustic rinse (e.g., 1–2% NaOH) is typically applied to neutralize residual acid and address any HCN that could be released by acid cleavage of cyanide complexes; one mine’s procedure uses 3–5% HCl for 1.5–2 h with agitation, followed by a water wash and then 1% NaOH to pH neutral (walkermining.com). Plants often meter these rinses with precision equipment akin to a dosing pump to hold pH and contact time steady.

Thermal regeneration kilns

Thermal regeneration follows the chemical clean to burn off remaining organics and restore micro-porosity. CIP plants generally batch-reactivate carbon every 5–10 adsorption/desorption cycles (walkermining.com). Rotary regeneration kilns are common: a first heating zone at ~600 °C drives off moisture and creates a steam-rich atmosphere; a second, hotter zone (typically 650–750 °C or higher) pyrolyzes and selectively oxidizes tarry residues via steam reactions (walkermining.com) (studylib.net).

Commercial kilns often operate around 650–800 °C; one operation describes a 600 °C zone and a >650 °C zone (walkermining.com), while some literature suggests pushing >800 °C for deeper carbon cleaning (studylib.net). Contact times run on the order of minutes and moisture loadings around ~50% are boiled off first (studylib.net) (walkermining.com).

Attrition and kiln design choices

Regeneration fires restore most of the carbon’s original activity — often 80–90% of virgin capacity when run effectively — but they also generate fines and burn-off losses (CO/CO₂), so limiting firing frequency is economical (studylib.net). Typical practice is to thermally regenerate only ≤50% of the carbon inventory per cycle; in effect, about half the carbon is held out of the kiln each shift, splitting the workload and lowering attrition (patents.google.com).

Kiln-induced attrition is a large slice of losses. Overall, one report assigns ~25 g per tonne of ore to kiln regeneration — about 40% of the 64 g/t total loss — with agitation and handling making up the other 60% (www.researchgate.net). Vertical electric kilns (“Minfurn”) have produced significantly less carbon fines than conventional rotary kilns, with one study finding the vertical design halved fines generation compared to a rotary unit (www.researchgate.net).

Specialty additives and alternatives

Beyond HCl and NaOH, nitric acid (HNO₃) is sometimes cited as an alternative for base metals, though it penetrates less into carbon pores (patents.google.com). Sulfuric acid is rarely used because it forms insoluble CaSO₄, except in proprietary methods. On the basic side, additional caustic washes — for example, NaOH/NaOCl — can help target grease; one patent notes a “caustic wash” step for organic contaminants (patents.google.com).

Oxidizing gases like ozone or hydrogen peroxide are commonly used in water-treatment carbon reactivation but are uncommon at CIP scale, where heating is typically sufficient because organics are minimal (studylib.net). In practice, the “specialty chemicals” in CIP focus on scale dissolution and pH control — HCl, NH₄HF₂, NaOH — and not on surfactants or chelating agents, since the main foulants are inorganic. Where rinse and neutralization streams need routing, plants fold them into existing water and wastewater chemicals strategies.

Outcomes and efficiency

Combined acid and thermal regeneration can recover most lost performance. The HCl/NH₄HF₂ wash method improved carbon activity by ~20–25% over untreated spent carbon (patents.google.com). After kiln reactivation, carbon usually returns near to virgin adsorption capacity; plant tails assays confirm ~0.1–0.5 g/t Au remaining, i.e., >99% recovery. Effective regeneration lowers consumption too: if a more aggressive acid wash adds 15–20% capacity, operators can run ~20% fewer kiln batches over time. Energy matters as well — coke furnaces or autoclaves can be energy-intensive — so reducing fuel use via steam injection, efficient burners, or electric kilns is an ongoing trend.

Regulations and waste integration (Indonesia)

In Indonesia, spent activated carbon containing adsorbed Au, heavy metals, and cyanide is classified as B3 (hazardous) waste. MOEF Regulation No. 9/2024 requires producers to minimize B3 waste and favor recycling or reuse, making on-site carbon regeneration both economically and legally important (www.hhp.co.id). Acid rinse brines and kiln off-gases are also regulated — including cyanide carbonation in the kiln stack — so CIP operations typically tie regeneration systems into wastewater treatment and gas scrubbing (e.g., converting HCN to ammonium thiocyanate). Plants often rely on wastewater ancillaries to handle these side streams within compliance.

Key figures and metrics

Typical CIP regeneration uses ~3–5% HCl by weight in acid washing (walkermining.com) (www.researchgate.net); washing times are ~1–2 h per layer. HCl concentrations up to ~10% have been studied, but 3–5% is common for safety and cost (studylib.net). After acid, rinsing with ~1% NaOH is used until neutral, avoiding residual cyanide being released as HCN (walkermining.com).

Kiln temperatures are ~600–750 °C, and carbon moisture loading is ~50%, which is boiled off first; approximately 5–10 adsorption cycles occur between each full regeneration (walkermining.com) (walkermining.com). Attrition rates of 60–100 g/t ore (combining all causes) are typical, so even small improvements — say, 10% less attrition — can save tonnes of carbon per year (www.researchgate.net). Overall, optimized acid washing and kiln control have restored carbon to >80–90% of its original capacity, supporting >95% gold recovery in CIP plants.

Sources and further reading

References include engineering texts and industry reports quantifying carbon use and restoration. Stanley (1987) reported 64 g/t carbon loss in CIP (www.researchgate.net). Patents and vendors detail wash chemistries such as HCl/NH₄HF₂ and NaOH rinses (patents.google.com) (walkermining.com), and operational studies quantify performance gains, such as 95%+ Ca removal by 3% HCl and ~20–25% capacity gains with optimized acid washes (www.researchgate.net) (patents.google.com). Guidance on kiln zones and regen frequency is available from plant guides (walkermining.com) (walkermining.com). Indonesian regulations (MOEF Reg. 9/2024) on hazardous B3 waste underscore the need for on-site reuse through regeneration (www.hhp.co.id). For pretreatment steps around carbon handling and auxiliary streams, some operators standardize on sitewide ancillaries akin to those used with water treatment ancillaries.