Placer Gold Processing: From Washing to Final Cleaning

Placer (alluvial) gold forms when gold-bearing rock weathers and the gold is carried and concentrated in river beds, terraces or slope deposits. Most of it is already liberated native gold with a density of roughly 15–19 g/cm³ (lower the more silver it contains), while quartz and other gangue minerals are only about 2.65 g/cm³. With such a large density difference, placer gold is recovered mainly by gravity separation and usually needs no crushing or grinding.

The hard part is rarely the separation itself; it is the washing and the catching. Clay can lock gold inside lumps, fine and flaky gold is easily lost with the water, and boulders and roots disrupt the equipment. A good flowsheet is built around those three problems. Alluvial gold deposits are common in places such as Kalimantan and Sumatra in Indonesia, and clay content and gold size vary a lot from one site to the next, so the flowsheet should always be matched to the ore.

Typical flowsheet

The diagram shows a general flowsheet; the actual set-up depends on the size distribution, clay content and gold characteristics of your sample. The sections below follow the process step by step.

1. Feeding, washing and screening

Run-of-mine material is loaded into a feed hopper by excavator or loader, usually through a grizzly that keeps out large boulders. Steady feeding is the foundation of a stable plant: if the feed surges and drops, every machine downstream performs worse.

Washing and screening are usually done in a trommel or rotary drum washer. The rotating drum tumbles the material while high-pressure spray water breaks up the clay and rinses gold off the gravel. The undersize slurry goes on to the next stage and the oversize gravel is checked and discarded. Screen apertures are typically 6–12 mm, chosen from the largest gold particles expected and the feed limit of the roughing equipment. Where clay is very sticky, a log washer or attrition scrubber ahead of the trommel gives the material more scrubbing time.

Trommel ScreenView equipment →Rotary Drum WasherView equipment →Log WasherView equipment →

Mobile trommel gold wash plant

2. Desliming

Fine slimes raise slurry viscosity, compact the jig bed and blur the bands on a shaking table, all of which cut gold recovery. A desliming cone or hydrocyclone is normally used to remove the finest clay. Fine gold can also report to the overflow, though, so sample and assay the slimes before fixing the cut size — do not throw gold away just to make the rest easier to treat.

3. Roughing: jigs, sluices and centrifuges

Roughing rejects most of the light sand and gravel while losing as little gold as possible. The usual options, used alone or in combination, are:

• Jigs — high capacity and a wide size range; well suited to coarser gold and heavy sand, and the most common roughing machine for placer gold.
• Sluices — simple and unpowered, with matting or riffles that trap the gold; good for small and mobile operations, but limited on fine gold and they need regular clean-ups.
• Centrifugal concentrators — centrifugal force boosts recovery of fine gold; often placed after the jig tailings or the sluice to catch fine gold the earlier units miss.

Spiral chutes also appear in placer flowsheets, mainly to pre-concentrate fine heavy sand (black sand). How well they hold flaky fine gold varies from ore to ore, so confirm it with testwork.

Trommel and sluice gold recoverySpiral chute separation circuit

4. Cleaning on shaking tables

Rougher concentrate still carries a lot of black sand (magnetite, ilmenite and so on) and other heavy minerals, so it needs cleaning. The shaking table is the usual tool: the gold shows up as a distinct line on the deck, and the operator sets the splitters by eye to take off a high-grade gold concentrate. Splitting the rougher concentrate into a coarse and a fine fraction before tabling usually improves results. For setting up a table, see How a Shaking Table Works and How to Adjust It.

Sawtooth-Wave JigView equipment →Centrifugal ConcentratorView equipment →6-S Shaking TableView equipment →

5. Final recovery: drying, magnetic removal and smelting

Table concentrate is dried, magnetic minerals such as magnetite are removed with a magnet, and the remaining concentrate is smelted with a flux such as borax to produce a crude gold bar. The whole route relies on physical separation and smelting — no mercury. Amalgamation is seriously harmful to people and waterways; Indonesia has ratified the Minamata Convention on Mercury and restricts mercury use in small-scale gold mining, so a new project should be designed mercury-free from day one.

6. Water recycling and tailings

Placer plants use a lot of water. The usual approach is a series of settling ponds or a thickener, so silt settles out of the tailings water and the clarified water is pumped back to washing and separation, with only a small top-up of fresh water. This saves water and keeps muddy discharge out of rivers. Tailings sand should go to a designated area so the land can be rehabilitated later.

7. Mobile or fixed plant?

Where the ground is scattered, each pit holds limited reserves and the plant has to follow the mining face, mobile equipment such as a diesel mobile trommel, paired with sluices or jigs, makes a self-contained unit that works without grid power. Where reserves are concentrated and the mine life is long, a fixed plant gives higher capacity, a more complete recovery circuit and a better water-recycling system. The two can also be combined: mobile washing at the front end, with rougher concentrate taken to a fixed cleaning room.

Diesel Mobile TrommelView equipment →Gold Washing PlantView equipment →Inclined-Tube ThickenerView equipment →

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