Adsorption: activated carbon and resin

Adsorption is the mechanism that built the modern gold industry and now reaches well beyond it. The idea is to load the dissolved value out of the solution or the slurry onto a solid — activated carbon or an ion-exchange resin — that can then be physically lifted out of the liquid and stripped of its load in a smaller, cleaner volume. It concentrates and purifies in one move: the value leaves a large, dilute, dirty stream and arrives on a solid you can carry, and that change of phase is the whole point.

Loading the value onto a solid

Activated carbon is a highly porous form of carbon with an enormous internal surface area; the gold cyanide complex adsorbs onto it strongly, and a tonne of carbon can hold kilograms of gold. An ion-exchange resin is a polymer bead carrying fixed charged groups that swap a harmless counter-ion for the dissolved metal complex. Both load the value from the solution onto the solid up to an equilibrium set by the solution tenor — the richer the solution, the higher the loading the solid reaches — and both are then separated from the liquid and stripped. The loading the solid carries, read against the tenor of the solution it sits in, is the central number of an adsorption circuit: it sets how much solid you must move and how rich the strip will be.

CIL, CIP, RIP and columns

The mechanism is arranged into a family of circuits by how the solid is brought to meet the liquid. CIP — carbon-in-pulp — adds carbon to a series of tanks of leached slurry, after leaching is finished, and adsorbs the gold from the pulp. CIL — carbon-in-leach — puts the carbon into the leach tanks themselves, so leaching and adsorption happen together; that is the route for a preg-robbing ore, where the carbon must capture the gold before the ore re-adsorbs it. RIP — resin-in-pulp — does the same with resin instead of carbon, where the resin’s selectivity or its tolerance of fouling wins. And fixed columns pass a clarified solution — not a slurry — up through a packed bed of carbon or resin: the arrangement for a clean leachate, as in heap-leach gold or uranium recovery. Slurry circuits keep the solid suspended in stirred tanks; columns hold it in a packed bed and pump the clear liquor through it.

The counter-current movement of carbon

The cleverness of a CIP or CIL train is the same counter-current trick as a washing circuit, run in adsorption — and it is the image to hold. The slurry flows down the train of tanks one way; the carbon is moved the other way, advanced stage by stage against the pulp. Fresh, barren carbon is added at the last tank, where the pulp is nearly stripped of gold; loaded carbon is pulled from the first tank, where it meets the richest incoming pulp. So the most loaded carbon meets the strongest solution and the barren carbon meets the weakest, and the train as a whole drives the dissolved gold in the pulp down to a low barren tenor while bringing the carbon up to a high loading. The carbon is screened from the pulp at each transfer and pumped or air-lifted forward against the flow: it is a moving inventory marching upstream, not a fixed bed sitting still.

The carbon inventory and sizing

Two quantities size such a circuit. The tanks are sized by residence time — the slurry’s contact time across the train, the same online-volume-over-flow calculation as a leach train — because the adsorption needs time to approach equilibrium, and the CIL-tank residence-time calculator below runs exactly that. The other quantity is the carbon inventory and its advance rate: how much carbon is held in the circuit and how fast it is moved against the pulp, which together set the loading the carbon reaches and the gold locked up in the circuit at any moment. The residence-time tool sizes the tankage here; the carbon-inventory balance — concentration, advance rate and loading profile — is a separate calculation this page names but does not yet land on a tool of its own.

Loaded carbon does not end the story: the value still has to come back off it and the carbon has to be made fit to reuse, which is the next topic. What adsorption has done is the hard part of purification — taken the value out of a large, dilute, dirty stream and put it on a solid small enough to handle.