In the gold mining process, milling is the stage where crushed ore is ground fine enough to expose gold for leaching or flotation. If you want to understand how gold is milled, think of it as controlled size reduction. Gold is rarely found as loose free metal; most gold is locked inside surrounding rock, often as tiny grains within sulphide minerals. Before gravity, flotation or cyanidation can reach that gold, the ore must be broken down to a particle size where the gold-bearing minerals are exposed. This is the job of the milling stage. A well-designed gold milling circuit ensures the recovery stages downstream receive a consistent, correctly sized feed. Get it wrong and you leave gold in the tailings regardless of how good the rest of the plant is.
Why milling matters in gold processing
Global gold mine production was an estimated 3,300 tons in 2024, according to the USGS Mineral Commodity Summaries. Very little of that came from coarse nuggets. The World Gold Council notes that gold mining operates on every continent except Antarctica, across mines of widely varying types and scale. Nearly all of them depend on milling to liberate gold from host rock. Without the right grind, gravity circuits miss fine gold, flotation leaves sulphide-locked particles uncollected, and cyanide cannot reach gold sealed inside unbroken grains.
The stages of a gold milling circuit
A typical gold milling circuit has three linked stages. Crushing uses jaw and cone crushers to reduce run-of-mine ore, often from hundreds of millimetres down to a coarse mill feed. Grinding uses rod and ball mills to reduce that feed further to the target grind size, usually expressed as P80. Classification uses hydrocyclones or spiral classifiers to split the mill discharge, returning oversize particles for another pass and passing correct-size particles to recovery. You'll find a deeper walkthrough in ore milling basics. Each stage is sized from ore testwork, never from a generic flowsheet.
Crushing: reducing run-of-mine ore
Run-of-mine ore often arrives at the plant with top sizes in the hundreds of millimetres. Jaw crushers handle coarse and intermediate crushing, while cone crushers take over for finer reduction through interparticle compression. Gyratory crushers suit very large primary duties, and high-pressure grinding rolls (HPGR) can replace or supplement conventional stages when ore hardness or energy costs demand a closer look. Hammer, impact and roll crushers are available for specific materials, but selection depends on top size, ore hardness and abrasiveness, moisture and clay content, target discharge opening, throughput, open or closed circuit, and wear-part availability. Standard catalogue parameters are based on typical materials; actual throughput shifts with your ore's characteristics and feed grading.
Grinding: ball mills and what drives selection
In a ball mill, the rotating shell lifts steel balls and ore, and the cascade of impact and attrition breaks particles down to the target size. Three common configurations appear in gold plants. Wet grate mills force discharge through a grate and suit primary grinding where fast removal matters. Wet overflow mills let slurry spill over the discharge end and are used for secondary grinding or finer products. Dry mills handle material without water and suit water-scarce sites or downstream dry processes. Xinhai builds ball mills up to 7 m in diameter for gold plants, and you can see how these are integrated in ball milling for gold processing.
| Ball mill type | Discharge behaviour | Typical position |
|---|---|---|
| Wet grate | Forced discharge through grate, lower slurry level | Primary grinding or circuits needing faster discharge |
| Wet overflow | No forced discharge; slurry overflows | Secondary grinding or finer products |
| Dry | Dry feed and discharge | Water-scarce or dry downstream circuits |
Selection also depends on Bond work index, feed and product size, slurry density, throughput, mill volume, motor power, liner and bearing configuration. The target grind size is normally expressed as P80: for example, a P80 of 75 µm means 80% of the ground ore is finer than 75 µm. That number drives energy calculation and mill sizing. A manufacturer may claim that rolling-element bearings save 20-30% energy compared with plain bearings; treat that as a vendor figure to verify against your own baseline and measured power.
Classification: closing the loop
Classification exploits differences in particle size, settling velocity or air drag to separate fine product from coarse returns. Spiral classifiers are simple and robust, and they work well at moderate cut sizes. Hydrocyclones use centrifugal force and are common in gold grinding circuits because they handle high throughput in a small footprint. High-frequency fine screens and air classifiers serve when tighter size control or dry operation is required. The key selection variables include cut size, feed pressure and density, slurry density, throughput, classification efficiency, overflow fineness, underflow density, wear materials and the number of parallel units. Oversize from the classifier returns to the mill, which is what makes the circuit a closed loop.
Testwork first, design second
You cannot size a gold milling circuit from a flowsheet alone. At Xinhai's CNAS-accredited laboratory, gold ores are characterised before any equipment is committed. Around 5,000 element analyses are performed each month, alongside Bond work index testing and recovery testwork for gravity, flotation and leaching routes. Bond work index measures how hard your ore is to grind, and that single number drives mill size, motor power and circuit configuration. Xinhai reports completing around 200 metallurgical tests per year, and the design institute holds a Class B metallurgical qualification. For more on ore characterisation, read how to test gold ore.
What “P80” means and how it sizes a mill
P80 is a standard mill-engineering definition: it is the screen aperture through which 80% of a ground sample would pass. If a circuit is designed to a P80 of 75 µm, then 80% of the product is finer than 75 µm and the remaining 20% is coarser. This single number controls three calculations. First, the target P80 sets the required energy input, which is estimated from the Bond work index in kilowatt-hours per tonne. Second, the P80 is matched to the recovery method: gravity circuits usually accept a coarser P80, flotation often targets 75–150 µm, and leaching routes are selected after testwork confirms how quickly gold is exposed. Third, the P80 determines the ball mill diameter, length, motor power and the number of classifier passes required. For example, Xinhai builds ball mills up to 7 m in diameter for gold plants; the correct diameter for a specific project is not picked from a catalogue but calculated from feed size, product P80, mill volume and measured work index. In a closed circuit, the classifier returns particles above the target P80 to the mill, while undersize flows to gravity, flotation or CIL/CIP recovery. That is why milling is described as controlled size reduction, not just crushing the rock finer.
From milling to recovery: choosing the route
Once the ore is ground to the right P80, gold is recovered by the route that fits the ore. Gravity separation captures coarse free gold early. Flotation concentrates gold-bearing sulphides. CIL and CIP cyanidation dissolve gold from the slurry, and on suitable ores recovery can reach 99% — but that is never a promise, only a testwork-verified ceiling. Heap leaching suits lower-grade or coarsely crushed material. Cyanide is toxic and must be handled under strict safety procedures; where applicable, operations follow the International Cyanide Management Code. Compare the main routes in gold recovery methods and the differences between CIL and CIP.
Getting the circuit right
Xinhai reports having served more than 2,500 mines and delivered over 600 EPC+M+O projects across more than 100 countries with a team of over 700 technical experts. Its three manufacturing bases cover roughly 110,000 m² and support more than 1,000 equipment types for plants up to 50,000 t/d. Those figures come from the company's published material, not from a third-party audit, but they tell you one thing: gold milling is not a catalogue pick. The right circuit starts with your ore, your target P80 and your recovery route, then works backwards to mill size and power. If you're designing a plant, talk to a team that tests before it quotes. See how a full plant comes together at gold milling plant solutions.