How Is Gold Extracted from Ore?

Most gold does not come out of the ground as a bright nugget. How is gold extracted from ore? It’s a sequence of size reduction, concentration, and chemical dissolution. The exact route depends on mineralogy, not on preference. A free-milling oxide ore may need only gravity and cyanide. A refractory sulphide ore may need flotation and oxidation before leaching. If you skip testwork, you risk building the wrong plant.

Metallurgical testwork defines the flowsheet

Before you buy a single pump, run a test program. The testwork answers three questions: how fine must you grind, which recovery method works, and what reagent consumption will be. A head assay quantifies gold grade in grams per tonne. A Bond work index test measures grindability. A gravity recoverable gold test shows how much gold can be captured by gravity. Leach bottle rolls show how much gold dissolves in cyanide over 24 to 72 hours. Flotation tests show if sulphide concentration is possible. This is the only reliable way to avoid building the wrong plant.

The extraction route starts with ore character

Before any equipment is selected, a metallurgical test program defines the ore. Gold can be present as coarse free grains, fine particles locked in sulphides, or submicroscopic gold inside pyrite or arsenopyrite. Each form points to a different flowsheet. A gold processing plant design is therefore built around grade, liberation size, and reagent response. The USGS Mineral Commodity Summaries describe how gold recovery depends on processing choices that vary by deposit type. That’s why testwork matters more than machinery.

Mining delivers the feed

The first step is getting ore to the surface. Open-pit and underground mining both produce a run-of-mine stream that varies in grade and hardness. You don’t want to feed a mill at random; blending and stockpiling smooth the grade. The plant’s capacity, measured in tonnes per day, fixes the size of every downstream machine. According to Xinhai’s published figures, its mining and processing equipment supports plants up to 50,000 t/d. That’s an enormous feed rate, but only after the ore is characterised should you commit to a specific flowsheet.

Comminution: breaking ore to liberate gold

Gold is useless until it’s liberated. Crushing and grinding reduce rock from boulders to particles fine enough for separation. The work index of the ore, measured by a Bond test, tells you how much energy the mills will consume. The Bond work index is a measure of the energy needed to grind a tonne of ore. Gold ore milling explains the trade-off between over-grinding and poor recovery. Comminution follows a standard sequence: primary crushing, secondary crushing, screening, coarse ore stockpile, then grinding in a SAG or ball mill. Cyclones classify the ground product, sending oversize back to the mill. The target grind size, often between 75 and 150 µm for gold, depends on liberation. Xinhai reports manufacturing mills up to a 7 m diameter ball mill, and a laboratory doing roughly 5,000 elemental analyses per month. That’s the kind of data you need to size a mill correctly.

Gravity recovery for coarse and free gold

If gold grains are coarse and free, gravity is the cheapest recovery method. Centrifugal concentrators, jigs, and shaking tables exploit the density difference between gold and gangue. Gravity can recover gold directly into a concentrate for smelting, or it can be used ahead of cyanidation to pull out coarse gold that would otherwise be slow to leach. Gravity recovery works in three broad steps: feed a classified stream to a centrifugal concentrator, collect a high-grade concentrate, and send the tailings to further leaching. The concentrate may be upgraded on a shaking table. A centrifugal concentrator can handle feed up to several hundred tonnes per hour, depending on model. The rule is simple: if you can see gold in a pan, gravity should be in the circuit.

Flotation for sulphide-hosted gold

Many gold ores are sulphide ores. Gold particles are too fine or locked inside pyrite or arsenopyrite, so gravity won’t work. Flotation uses chemical reagents to make gold-bearing sulphides attach to air bubbles and float off. The result is a sulphide concentrate that can be sold to a smelter or treated further by leaching. A gold flotation plant is designed around flotation cells, regrind mills, and thickeners. Xinhai reports manufacturing flotation cells up to 320 m³ per unit and thickeners up to 100 m in diameter. Such equipment serves large-scale operations where copper or arsenic must be separated from gold. Flotation does not dissolve gold. It concentrates gold-bearing sulphides into a smaller mass that can be processed more economically. The tailings from flotation may still contain free gold, so some plants combine flotation with gravity or cyanidation.

Cyanidation: CIL, CIP, and heap leaching

For most free-milling and oxidised gold ores, cyanidation is the workhorse. Gold dissolves in a weak cyanide solution and is then recovered onto activated carbon. CIL stands for carbon-in-leach, where leaching and carbon adsorption happen in the same tanks. CIP means carbon-in-pulp, where leaching finishes before carbon contacts the slurry. CIL or CIP recovery can reach 99% on suitable free-milling ores. That’s not a guarantee for every ore, but it’s a realistic target for simple oxides. Compare CIL vs CIP before choosing tank layout. A CIL circuit has these steps: leach tanks in series, carbon added to each tank, carbon moves countercurrent to the pulp, loaded carbon is screened out, and gold is stripped from carbon. CIP separates leaching and adsorption into different tank sets. For low-grade ore or ore stacked on a pad, heap leaching is a lower-cost option. Leach solution percolates through the heap and gold is recovered from the pregnant solution. According to the International Cyanide Management Code, cyanide use must be managed with strict safety and environmental controls. Refractory ores are those where direct cyanidation recovers only a small fraction of the gold because the gold is locked in sulphide minerals. Such ores need pre-treatment such as roasting, pressure oxidation, or bio-oxidation.

Refining and final gold product

After leaching or flotation, the gold-bearing solution or concentrate is processed further. The classic step is electrowinning or zinc precipitation to produce a gold sludge, followed by smelting into doré bars. Doré is a semi-pure alloy of gold and silver that refineries accept. Electrowinning uses an electric current to plate gold from solution onto steel wool cathodes. Zinc precipitation, also called the Merrill-Crowe process, uses zinc dust to cement gold out of solution. Both produce a sludge that is smelted into doré. Refining to market-grade gold happens off-site at a refinery. The World Gold Council describes how doré moves from mines to refineries and then to market. For mine owners, the final product is usually doré or gold concentrate, not bullion.

How to choose the right route

You don’t choose based on what worked at another mine. You choose after testwork. A proper test program includes head assays, Bond work index tests, gravity recoverable gold tests, and leach bottle rolls. Xinhai reports its CNAS-accredited laboratory runs roughly 200 beneficiation tests a year, using industrial-scale pilot equipment. According to Xinhai’s published figures, the company has served over 2,500 mines and 600+ EPC projects across 70+ ore types. That’s useful when you’re scaling from drill core to a gold recovery plant. The goal is to define a flowsheet that maximises recovery without over-building. For a small deposit, a modular plant may be enough. For a large refractory deposit, flotation plus pressure oxidation may be required. The ore decides.

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