Carbon Activity Testing for Reliable Gold Recovery in CIL and CIP Circuits

For operators of carbon-in-leach and carbon-in-pulp plants, the gap between expected and recovered gold often comes down to the unseen condition of the activated carbon moving between tanks. Carbon activity testing provides a numerical bridge between laboratory ore characterisation and the day-to-day decisions made on the leach pad, in the elution column and across the regeneration kiln. The technique is not glamorous — it is a bench-scale measurement — yet it quietly underwrites the gold room's reconciliation every month.

Across Australia, where mills in Kalgoorlie, the Boddington corridor and the Eastern Goldfields process everything from free-milling oxide ores through to refractory sulphide blends, the discipline of measuring carbon condition has become part of the monthly metallurgical rhythm. The test helps engineers decide when to trim cyanide, when to advance regeneration cycles and when to swap in fresh carbon. Done well, it turns the carbon inventory from a passive sorbent into a measurable process variable that can be dialled, audited and improved.

Why carbon condition drives plant economics

Activated carbon in a leach or adsorption circuit is constantly loaded, stripped and reactivated. With each loop, microscopic pores that once trapped gold-cyanide complexes accumulate fine silica, flotation reagents, organic matter and carbonate scale. The result is a slow drift in the carbon's capacity to grab gold from solution, even when the kiln is firing at the right bed temperature. Activity testing is simply the controlled measurement of that drift against a known reference material.

The financial impact is rarely visible on a single shift but compounds across campaigns. A one hundred gram per tonne drop in equilibrium gold loading can quietly bleed five to eight percent of plant recovery, depending on the ore type and residence time. Engineers in Western Australia who reconcile month-end figures have learned that inconsistent carbon activity is frequently the missing variable once mass pulls are balanced and screen inventories are checked.

How operators sample loaded and regenerated carbon

Reliable results begin at the sampler. A representative scoop of carbon is taken from the inter-tank screen of the CIL train, rinsed in site water to remove slurry fines, and sealed in a labelled HDPE bottle. Many Australian sites twin this with a second sample drawn from the regenerated carbon transfer launder, so the activity of fresh and loaded carbon can be compared on the same shift.

Back at the lab, the wet carbon is screened to remove wood chips and broken fragments, then dried to a constant mass. A sub-sample is typically acid-washed to strip residual iron and calcium, after which it is rinsed, re-dried and cooled in a desiccator. This preparation is mundane but important: any residual moisture or scale left on the grains will distort the equilibrium reading and produce misleading recommendations further downstream.

Running the equilibrium and kinetic test

The prepared carbon is contacted with a synthetic gold-bearing solution of known concentration under controlled pH, temperature and alkalinity. The Australian standard approach uses a cyanide-stabilised liquor at around 800 ppm NaCN, pH 10.5, held at the slurry temperature of the host plant. Aliquots are withdrawn at defined intervals — usually one, four and twenty-four hours — to define both the rate of adsorption and the equilibrium ceiling.

Plotted values yield two figures the metallurgist cares about: the kinetic rate constant, often called the K-value and expressed as h⁻¹, and the equilibrium loading, expressed as grams of gold per tonne of carbon. A fresh batch of coconut-shell carbon should benchmark at roughly 950 to 1100 g/t equilibrium loading, with a strong kinetic slope over the first hour. Anything materially below that benchmark signals either regeneration shortfall, organic fouling or simple attrition losses from inter-stage pumping.

What shifts carbon activity between campaigns

Regeneration is the biggest single lever. A kiln running cool, an under-sized regeneration furnace or extended residence times all degrade the carbon's microporosity over weeks rather than days. Operators who map activity against kiln temperature and bed residence time usually find a clean correlation that allows them to push throughput without sacrificing capacity.

Feed-side disturbances matter just as much. Preg-robbing shales, graphitic schists and ores with high organic carbon content can chemically out-compete activated carbon for gold-cyanide, leaving the carbon itself under-loaded yet apparently "active". Transitional ores in the Eastern Goldfields frequently present this pattern during the spring wet season, when groundwater brings fresh organic loads into the pit. Routine activity testing catches the shift early and points operators toward blending strategies or pre-oxidation steps before recovery is eroded.

Linking activity readings to gold recovery and reagent use

A single set of activity numbers only earns its keep when it is plotted against recovery, cyanide consumption and elution efficiency. Most Australian plants now maintain a small dashboard that overlays monthly activity readings with the locked-cycle recovery curve. When equilibrium loading softens, sodium cyanide dose and slurry dissolved oxygen are the first knobs to turn; if those moves do not arrest the slide, the conversation moves to a carbon top-up and a closer look at the regeneration circuit.

Some sites also use activity results to size and time the addition of fresh carbon. Rather than discarding the entire inventory on a fixed schedule, operators bleed a portion of the lowest-activity carbon each month and replace it with virgin or fully reactivated material. The strategy trims operating cost, reduces elution load and stabilises recovery during ore transitions that are common in open-pit operations around Kalgoorlie and the Tanami.

Embedding activity testing into a wider metallurgical programme

Activity data is most powerful when it feeds back into broader test work. Composite samples from drill core, geometallurgical blocks and operating campaigns can be leached with a defined carbon envelope to predict how a future ore blend will behave. Locked-cycle testing that includes carbon activity monitoring is now standard for feasibility work on Australian greenfield projects, particularly where oxide-to-sulphide transitions are forecast within the first five years of mine life.

For plants already in production, integrating activity readings with ore characterisation, mineralogy and slurry rheology unlocks process improvements that no single dataset could deliver on its own. Engineering teams that bundle this analysis with pump selection, tank sizing and instrumentation often shorten the path from sample to operating gain. Site operators struggling with tailings disposal or thickener performance sometimes find that the same diagnostic discipline, applied to slurry transport, exposes hidden bottlenecks. A practical reference for high-head applications is the article on Selecting Centrifugal Pumps for High-Head Slurry Transport, which covers head, efficiency and wear considerations relevant to dense slurries carrying fine carbon.

Australian notes from oxide, transitional and sulphide operations

The climate shapes how activity data is interpreted. In the Pilbara and the Goldfields, summer temperatures push slurry tanks above 35 °C for several months of the year, accelerating both adsorption kinetics and, unfortunately, carbon attrition. Sites that work through the wet season in the Northern Territory see the opposite: cooler slurries slow kinetics enough that operators must extend CIL residence time or accept a lower equilibrium loading on each campaign.

Workforce patterns also matter. The fly-in fly-out rhythm common across Australian mining means that key laboratory technicians are often on site for only a portion of the roster, so activity testing protocols need to be robust enough to be executed by rotating staff without compromising consistency. Several operators in Western Australia have addressed this by centralising activity work in a hub laboratory in Perth or Brisbane, with samples couriered weekly from regional plants, while keeping rapid turn-around screening tests on site for day-to-day decisions.

Carbon activity testing is a small laboratory routine with out-sized financial consequences, and it sits naturally inside a wider offering of process engineering, equipment supply and operational support. Plants that combine monthly activity programmes with periodic ore testing, plant audits and EPC services tend to recover more gold from the same ore body while consuming less reagent and producing less loaded carbon waste.

For project owners evaluating new circuits, expanding capacity or troubleshooting an existing CIL or CIP train, the next step is to review turnkey plant solutions that bring together metallurgical testing, equipment selection and commissioning in a single scope. Reach out to the engineering team to scope a fresh activity baseline, an audit of the regeneration kiln, or a broader plant redesign tailored to the ore body and the local operating conditions.