Gold Leaching: Daily Control for Contract Operators

What gold leaching actually is (and where it fits in a gold plant)

Say a mine owner asks why two plants with identical cyanide tanks can deliver very different gold recoveries. The answer rarely sits in the equipment list. Gold leaching is a hydrometallurgical step that dissolves gold from ground ore into solution, and it's the operator's control of density, pH, cyanide and oxygen that separates a steady circuit from one that swings. You'll see the same tanks, the same carbon, even the same reagent addition points. The difference is what happens on shift.

Leaching sits after grinding. You grind the ore fine enough to expose gold surfaces, then you mix that pulp with a dilute cyanide solution in a series of agitated tanks. Gold dissolves into the liquid. From there, the pregnant solution goes to either carbon adsorption or zinc precipitation. Carbon is the more common choice in modern CIP and CIL plants. If you're looking at the equipment side, gold processing solutions lists the main machines.

Low-grade ores matter here. You can't economically smelt a low-grade feed, but you can leach it if the operator holds the circuit steady. Overall plant recovery often depends more on day-to-day leach control than on the design recovery in the feasibility study. That's the operator's field.

Cyanide leaching chemistry and the Elsner equation

Cyanide dissolves gold in the presence of oxygen. The Elsner equation, 4 Au + 8 CN^- + O₂ + 2 H₂O = 4 Au(CN)₂^- + 4 OH^-, shows oxygen is not optional. You need dissolved oxygen as a co-reagent. Without enough oxygen, the reaction slows even if free cyanide is plentiful.

That's why an operator watches dissolved oxygen in the leach tanks. Air spargers, oxygen injection, or simply open-tank agitation can lift it. Cyanide Facts gives a typical operating range of 300 to 500 mg/l for cyanide concentration, though the right figure depends on mineralogy. You'll also hold pH alkaline, usually with lime. An alkaline pH keeps cyanide mostly as free cyanide rather than hydrogen cyanide gas. HCN gas is both a safety problem and a reagent loss. The same source makes that clear.

Don't chase a single number across every ore. Testwork defines the concentration, oxygen demand and lime dose for your ore. The operator's job is to maintain what the metallurgist specified.

Leach feed preparation: density, grind size and pH control

Leach feed preparation is where many circuits quietly drift. Grinding controls liberation and particle size. P80 is the size 80 per cent of the mass passes. A coarser grind leaves gold locked in gangue. A finer grind may consume more power and reagents than the recovery gain justifies. The laboratory sets the target. The operator holds the mill product close to it.

A US EPA summary notes ground ore is typically 60 to 80 per cent finer than 200 mesh (74 microns) in gold milling (Nevada Gold Cyanide Mill Tailings Regulation). That's not a target for every plant, but it shows the order of magnitude. Oxidised ores can behave differently; see this note on oxidised gold ore.

Leach feed density is a lever on residence time and reagent consumption. If the pulp is too thick, mixing suffers and oxygen transfer drops. If it's too thin, your tanks give you less effective residence time per unit of solid, and reagent cost per tonne of ore climbs. You'll normally run a density set by testwork, then measure it every shift. A Marcy gauge or a nuclear density meter is standard.

pH control matters because cyanide stability depends on it. Lime slurry is the workhorse. Add it to the grinding circuit or the first leach tank, and monitor pH online. If pH falls, cyanide can report to the gas phase. If pH climbs too high, you might scale lines and increase lime cost without a metallurgical benefit. It's a narrow operating window.

CIP vs CIL: carbon management in a running leach circuit

CIP and CIL differ in where adsorption happens. CIP keeps leaching and adsorption in separate tank sets. CIL combines them in the same tanks. The choice usually follows ore behaviour, not fashion.

Carbon concentration is a daily operating variable. Too little carbon and dissolved gold leaves in tailings. Too much carbon and you spend more on elution, sizing and make-up carbon without speeding up adsorption enough. Counter-current transfer works by moving carbon against the pulp flow: the freshest carbon meets the lowest-grade solution, and loaded carbon leaves from the first contactor. That maximises the loading on each tonne of carbon.

Screening matters more than it looks. Interstage screens keep carbon in its tank while letting pulp pass. If screens blind, pulp flow backs up or bypasses. If screens tear, carbon migrates downstream and you lose gold-loaded carbon to tailings. Carbon losses are not just a consumable cost; they represent gold inventory leaving the circuit.

What goes wrong in a contract-operated leach circuit

High soluble gold in tailings is the classic alarm. It tells you the gold was dissolved but not adsorbed. That could mean too little carbon, poor carbon activity, short-circuiting in the adsorption tanks, or a screen failure sending loaded carbon to tailings. You don't fix it by adding more cyanide. You sample the tailings, check the carbon concentration, and inspect the screens.

Preg-robbing is a nastier problem. Some ores contain carbonaceous matter that competes with activated carbon for the gold cyanide complex. CIL is often chosen for these ores because the activated carbon is present where gold dissolves, but preg-robbing can still beat you if the carbon is fouled or the ore is more active than testwork suggested. Operators diagnose preg-robbing by comparing soluble gold in a standard leach with and without added carbon. If the without-carbon test loses gold from solution, the ore is robbing it. The fix may be more carbon, finer carbon, or a blinding agent, or in severe cases a different flowsheet.

Carbon fouling is another common fault. Fine particles, oils, or organic matter coat the carbon surface and block the pores. It shows up as falling gold loading per tonne of carbon even when soluble gold in solution is high. The circuit may need more frequent reactivation or acid washing. Screen blinding often accompanies fouling: fines and scale build up on the wedgewire, reducing open area. You'll see level swings in the tanks and carbon carryover. The operations team can backwash screens, adjust carbon advance rates, or add a trash screen upstream, all without stopping production.

You diagnose before you change anything. That's the operator's discipline. Sample the feed, the leach tail, the loaded carbon, the barren solution. Check pH, free cyanide and dissolved oxygen at the point where they matter. Then make one change, not five.

Cyanide handling, compliance and the ICMI Cyanide Code

Cyanide is toxic. That's not a phrase to soften. It requires engineered controls, written procedures and a trained crew. The Cyanide Code provides a management system for safe use of cyanide through its entire life cycle. It's voluntary, but many serious operators use it as a benchmark.

An operating site under the Code won't just have a manual on a shelf. It'll have defined responsibilities, regular training, cyanide inventory control, emergency response equipment and clear monitoring. A code-compliant site will have multiple cyanide related procedures, not just one.

In daily operation, a contract operator embeds cyanide safety into production management. The shift crew knows the alarm values for HCN gas, how to respond to a pH excursion, and where the cyanide antidote kit sits. They check bunds, sumps and lines. Safety is not a separate department; it's the same set of checks that keeps recovery steady. A cyanide spill and a leach upset often start from the same root cause: a pump failure, a blocked line, or a bad pH reading.

Operating a gold leach circuit under EPC+M+O

The O in EPC+M+O stands for operation. It covers production, equipment, safety, environment, people and finance. In a gold plant, that means the leach circuit is not just a black box; it's a managed process with daily targets and accountabilities. Xinhai reports more than 600 mine EPC+M+O projects, according to the company's published figures. That breadth matters because a contract operator has seen the same failure modes across many ores and climates.

Contractual models align leaching performance with operator accountability. A KPI turnkey model sets targets for throughput, recovery or availability. A fixed-cost contract gives the owner a predictable cost, but the operator still carries performance risk. A cost-plus-profit-share model links the operator's margin to incremental recovery or production, which concentrates minds on stable cyanide and oxygen control. Xinhai has delivered a 4,000 t/d gold carbon-in-pulp plant in Kyrgyzstan. That project shows the EPC side feeding directly into the O side: the same team that designed the leach tanks then runs them.

Metallurgical testwork and design feed stable operation. Xinhai operates a CNAS-accredited laboratory (ISO/IEC 17025) under Yantai Xinhai Mining Research & Design Co., Ltd. That lab defines the leach residence time, the grind target, and the reagent suite before the plant is built. When the ore changes, the operator has a baseline to work from. You can't operate what you haven't measured. See the EPC+M+O service model for how the phases connect, and the project portfolio for examples.

FAQ: gold leaching questions from site

What is leaching gold?

Gold leaching is a hydrometallurgical process that dissolves gold from finely ground ore into a water-based solution, usually with dilute cyanide and dissolved oxygen. The gold forms a soluble complex, which is then recovered by carbon adsorption or zinc precipitation. It's the step between grinding and gold recovery.

What chemical is used for gold leaching?

Sodium cyanide is the chemical used in commercial gold leaching, usually in an alkaline solution. It reacts with gold and dissolved oxygen to form a soluble gold-cyanide complex. The Elsner equation describes the reaction. Operators hold the cyanide concentration, pH and dissolved oxygen in the ranges set by testwork.

What is the difference between CIP and CIL in gold leaching?

CIP separates leaching and carbon adsorption into different tank sets, while CIL runs both in the same tanks. CIL is often chosen for preg-robbing ores because the activated carbon competes with the ore's natural adsorbents immediately. CIP gives more independent control of leach residence time and carbon handling.

How do operators control cyanide and oxygen in a gold leach circuit?

They keep pH alkaline, usually with lime, so cyanide stays as free cyanide and not hydrogen cyanide gas. They monitor free cyanide concentration by titration or online analyser, and add cyanide to hold the testwork target. Dissolved oxygen is maintained with air sparging or oxygen injection, and is checked in the leach tanks. If free cyanide is adequate but leaching slows, the first check is often dissolved oxygen.

Frequently asked questions