What ore milling is
Gold processing steps start far earlier than the leach tanks. They start in the milling circuit, where mined rock is crushed and ground until the valuable gold particles are liberated from gangue. Ore milling refers to the physical reduction of mined rock into fine particles so that individual mineral grains are free from surrounding waste rock. The milling stage is the link between the mine and the recovery plant, and its design determines how much gold you can actually recover downstream. This page walks through the basics and shows where milling sits in a gold processing flowchart.
Most valuable minerals are locked inside waste rock. Liberation is the goal, but size reduction costs money. Every kilowatt-hour you spend grinding is a cost against the value of the recovered gold. So you need a circuit that reaches the right grind size without wasting energy.
Liberation: the core target
Liberation means breaking ore down until individual mineral grains are free of the surrounding rock. The finer you grind, the more mineral you liberate. However, grinding finer also costs more energy and can create slimes that are hard to recover in downstream flotation or leaching. The target is the coarsest size that still liberates enough mineral, not the finest possible grind.
That target differs for every ore type. A coarse-grained free-milling ore might liberate gold at 150 µm, while a refractory ore with fine inclusions may need 38 µm or less. You find the target through testwork, not through guesswork. Once you know the required P80, you can size the mills, motors and classification equipment.
Grind size and the Bond work index
Grind size is often expressed as P80 — the size at which 80% of the material passes. A P80 of 75 µm means 80% of the ground product is finer than 75 micrometres. This single number sets the performance target for the whole grinding circuit.
The energy needed to reach a given grind size is measured by the Bond work index. It's a laboratory test that quantifies how hard the ore is to grind. A low Bond work index means the ore breaks easily; a high one means you'll burn more energy per tonne. That number sizes the mill, the motor and the circuit configuration. Xinhai's laboratories run Bond work index tests as part of ore characterisation for gold milling plant design.
Many gold plants target a P80 between 75 µm and 150 µm for leaching, but gravity circuits can operate coarser. A finer grind usually improves recovery but raises energy cost; the optimum is where the marginal recovery value equals the marginal grinding cost.
Gold processing flowchart: where milling sits
A basic gold processing flowchart moves through five stages. Step 1: crushing reduces run-of-mine rock to a size that grinding can handle. Step 2: grinding reduces the crushed product to the liberation size. Step 3: classification separates the ground material by size, returning oversize to the mill. Step 4: concentration uses gravity or flotation to pull gold-bearing particles into a smaller stream. Step 5: leaching, often with cyanide, dissolves the remaining gold for recovery. Not every gold ore uses all five steps, but the milling section covers steps 1 to 3.
Cyanide is toxic, and its use is governed by compliance and safety procedure, not by a catalogue. The International Cyanide Management Code describes a voluntary industry programme covering production, transport, storage and use of cyanide. When ore suits cyanidation, a CIL or CIP circuit can reach 99% recovery on suitable ores. If you need help selecting between the two, see CIL vs CIP.
The flowchart is not fixed. Some ores get gravity concentration before leaching; others use flotation to make a sulphide concentrate. The milling circuit stays the same in principle, but its configuration shifts with the recovery route.
Crushing: the first size reduction step
Crushing takes run-of-mine rock down to a size that grinding mills can accept. The equipment family includes jaw crushers for coarse crushing, cone crushers and gyratory crushers for secondary and tertiary stages, and high-pressure grinding rolls for certain hard rock applications. Selection depends on the ore's maximum particle size, hardness, abrasiveness, moisture content, target discharge setting, hourly throughput and whether the circuit runs open or closed.
Cone crushers often run in closed circuit with screens. The screen returns oversize to the crusher, which keeps the product from drifting above the mill feed size. Xinhai's gold milling plants configure crushing and grinding as one integrated stage, not as separate islands. For a closer look at the whole milling sequence, see how gold milling works.
Grinding mills: choosing the right machine
Grinding mills do the fine reduction. The main choices are ball mills, rod mills, autogenous mills and vertical stirred mills. Among ball mills, three types dominate: wet grate, wet overflow and dry. The table below summarises where each type fits.
| Ball mill type | Discharge characteristic | Typical position |
|---|---|---|
| Wet grate discharge | Forced discharge through grate; lower slurry level | First-stage grinding or when faster discharge is needed |
| Wet overflow discharge | No forced discharge; slurry overflows at the end | Second-stage grinding or finer product |
| Dry ball mill | Dry feed and discharge | Water-scarce sites or dry downstream processes |
Ball mill selection also depends on shell diameter and length, effective volume, installed motor power, maximum ball charge, feed and product size, pulp density, liner material and bearing configuration. Xinhai's equipment range includes ball mills up to 7 m in diameter, suitable for high-tonnage gold plants.
Rod mills use long steel rods instead of balls. They produce a more uniform product and reduce overgrinding in the coarse end, so they often sit before a ball mill in a two-stage circuit. Vertical stirred mills are used for fine and ultrafine regrinding, often before flotation or leaching. For more on ball mill choices, read ball milling for gold processing.
Classification and closed circuits
Classification separates fine product from coarse oversize. The main machines are hydrocyclones, spiral classifiers, high-frequency screens and air classifiers. A closed grinding circuit returns oversize particles to the mill while allowing correctly sized material to continue downstream. Closed circuits improve grinding efficiency because the mill only does useful work on particles that still need breaking.
Circulating load is the mass of oversize returned to the mill divided by the mass of fresh feed. It is set by testwork; a higher circulating load can improve efficiency but increases pump wear. Selection variables include the cut size, feed pressure, slurry density, throughput, classification efficiency, overflow fineness, underflow density, wear materials and the number of parallel units. You don't pick a hydrocyclone from a chart alone; you match it to the circulating load and the target P80.
Need a complete circuit designed from test data? Our gold processing plant solutions start with ore characterisation, not catalogue assumptions.
Testwork, design data and project standards
You can't size a milling circuit properly without measuring how the ore behaves. Xinhai reports that its CNAS-accredited laboratory runs about 5,000 element analyses per month and about 200 beneficiation tests per year. That includes Bond work index tests, batch grinding tests and flowsheet development at pilot scale.
Xinhai also reports a project record of 2,500+ mines served and 600+ EPC M+O projects across 100+ countries. Published gold project references include a 15,000 t/d gold processing EPC in Guinea and a 4,000 t/d gold carbon-in-pulp plant in Kyrgyzstan. These are company-reported figures, and they tell you the engineering group has seen more than one ore type.
For projects that will report resources or reserves, design data often needs to align with standards such as the JORC Code or NI 43-101. Shandong Xinhai's design institute holds a Class B metallurgical qualification in China; that class does not automatically apply in other jurisdictions.
Test data beats catalogue data. If you're at the testwork stage, see how to test gold ore before you commit to equipment.