What gold ore milling does
Gold ore milling is the size-reduction step that sits between crushing and the recovery circuit. Its job is not to dissolve or separate gold; it is to liberate gold-bearing minerals from gangue so a downstream process can collect them. A common target for many sulphide ores is a P80 of 75 µm, which means 80 per cent of the milled product will pass a 75 micrometre screen. Oxide ores often need a coarser or finer target depending on how the gold is hosted. Xinhai's design approach, described in how gold milling works, starts with that liberation target before equipment is selected.
Oxide versus sulphide gold ores
Gold ores split broadly into oxide and sulphide types. Oxide ores are weathered near-surface materials; gold is often free or only lightly locked, and they usually respond to gravity or direct cyanidation. Sulphide ores come from deeper zones, with gold trapped inside pyrite or arsenopyrite. They commonly require a finer grind and a flotation step before leaching. The ore type therefore drives the grind target, and a deposit should never be milled on a guess. Worldwide gold mine production was an estimated 3,300 tons in 2024, according to the U.S. Geological Survey. Xinhai's gold flotation plant solutions address the sulphide case, while oxide circuits may move straight to leaching.
The milling circuit: crushing, grinding, classification
Gold ore crushing is the first stage of the size-reduction chain. Jaw crushers and cone crushers take run-of-mine rock down to a feed size a ball mill can handle. From there, grinding in rod mills or ball mills takes the ore to the target P80. Classification then closes the loop: hydrocyclones split the mill discharge, returning oversize for another pass while sending correctly sized material to recovery. A plant treating 1,200 t/d will typically run the crusher and mill in closed circuit with cyclones to avoid overgrinding. Xinhai supplies the full circuit, including ball mills up to 7 m in diameter, from gold milling plant designs.
Crushing equipment choices for gold ore
Gold ore crushing starts the size-reduction chain. The main equipment families are:
- Jaw crushers for primary crushing or medium-to-fine crushing of hard rock.
- Gyratory crushers for large, continuous primary crushing duties.
- Cone crushers for secondary and tertiary crushing using interparticle breakage.
- Hammer, impact and roll crushers selected by ore hardness and target product size.
- High-pressure grinding rolls (HPGR) for high-pressure particle-bed breakage when testwork supports it.
Selection depends on run-of-mine top size, ore hardness and abrasiveness, moisture and clay content, target discharge setting, hourly throughput, and whether the circuit is open or closed. Catalogue figures are standard machine data only; actual capacity changes with feed grading and ore character, so final selection must rest on testwork and a confirmed process calculation.
Ball mill types and where they fit
Ball mills do most of the fine grinding in gold circuits. Three types dominate: wet grate, wet overflow and dry. Wet grate mills use a grate to force discharge and keep the slurry level lower; they suit primary grinding or circuits needing fast removal of ground material. Wet overflow mills have no grate; slurry leaves by overflow, so they fit secondary grinding or fine product targets. Dry ball mills feed and discharge dry material and are chosen where water is scarce or the downstream process is dry. The equipment manual notes that a rolling bearing design can cut energy use by 20–30 per cent compared with a conventional sliding bearing, but that figure is a vendor claim and should be checked against your ore and measured power draw. Learn more about the mill choices in ball milling gold processing.
| Type | Discharge behaviour | Typical placement |
|---|---|---|
| Wet grate | Forced discharge, lower slurry level | Primary milling or fast discharge |
| Wet overflow | No forced discharge, overflow leaves mill | Secondary milling or finer product |
| Dry | Dry feed and discharge | Water-limited or dry downstream flows |
Grind size comes from Bond work index testing
Before any mill is sized, Xinhai's laboratory runs a Bond work index test. The result is a number expressed in kilowatt-hours per tonne (kWh/t) that estimates the energy required to grind your ore from a coarse feed to a target product size. That single figure drives mill motor power, mill dimensions and circuit configuration. According to the company's published figures, Xinhai's CNAS-accredited laboratory handles around 5,000 element analyses per month and about 200 metallurgical test programmes per year. A high Bond work index pushes a project toward more grinding power or a staged circuit; a low value can allow a smaller mill. You can see how ore testing fits the design flow in how to test gold ore.
Classification and closed-circuit grinding
Hydrocyclones are the workhorses of gold mill classification. They use centrifugal force to separate fine product from coarse oversize. Key selection variables include cut size, feed pressure, slurry density, throughput and wear material. A closed circuit returns the cyclone underflow to the mill, which increases the amount of correctly sized material leaving to recovery. If the cut point is too coarse, gold-bearing sulphides stay locked and recovery falls; if it is too fine, the mill grinds slime and power cost rises. Xinhai's equipment range covers hydrocyclone series including XC I, XC II, XC III, XHCV and XN, matched to the mill and the target P80. For the underlying concepts, see ore milling basics.
Gold recovery routes after milling
After milling, gold recovery takes one or more of four routes: gravity, flotation, cyanide leaching and heap leaching. For free-milling oxide ores, gravity or direct cyanidation can work well. For sulphide ores, flotation often produces a concentrate that is then leached. Carbon-in-pulp and carbon-in-leach circuits, compared in gold CIL vs CIP, are common for finely ground ores. On suitable ores, CIL/CIP recovery can reach 99%, but that figure is a design target, not a guarantee. Cyanide is toxic; its transport, storage and use are governed by safety systems and, where adopted, the International Cyanide Management Code. The International Cyanide Management Code is a voluntary industry programme covering the production, transport, storage and use of cyanide, with certified operations audited by third parties.
Xinhai's gold milling capability
Founded in 1997, Xinhai reports that it has served more than 2,500 mines and delivered over 600 mine EPC+M+O projects across 100+ countries, with a technical staff of more than 700 experts. Its three manufacturing bases cover about 110,000 m² and support more than 1,000 equipment types for plant capacities up to 50,000 t/d. For gold specifically, Xinhai can supply gravity, flotation, CIP/CIL and heap leach circuits. Its design institute holds a Class B metallurgical qualification in China; general contracting for mine construction is covered by a Class A qualification. Approved reference cases include a 15,000 t/d gold processing EPC project in Guinea, gold mining and processing contracting in Ghana, and a 4,000 t/d carbon-in-pulp plant in Kyrgyzstan. See the Ghana gold plant and Kyrgyzstan gold plant pages for regional detail.