Gravity Concentrating Table

Overview

A gravity concentrating table, often called a shaking table or Wilfley table, separates fine heavy minerals from lighter gangue using the difference in their density. It is a slightly inclined rectangular deck covered with low riffles. The deck is given an asymmetric horizontal shake while a thin film of water flows across it at right angles to the shake. Heavy grains settle behind the riffles and are carried along the deck by the shake toward the Concentrate Launder, while light grains stay high in the film and are washed over the riffles to the Tailings Launder. The result is a fanned set of bands across the discharge edge, from clean concentrate at one corner to tailings at the other.

The machine has eight subsystems on one base. The Concentrating Deck does the separation, the Head Motion Mechanism supplies the asymmetric shake, the Wash Water System lays the film, the Deck Suspension carries and tilts the deck, the Product Collection catches the products, the Drive Train turns the eccentric, and the Frame and Base holds it all level, with the Control System setting speed, slope and water.

The deck and riffles

The Concentrating Deck is the working face. A Backing Panel, usually laminated marine plywood or welded steel, gives a flat, rigid backing, stiffened underneath by Cross Stiffener ribs so it does not bow. A Deck Wear Surface of bonded rubber, fibreglass or linoleum is the low-friction face the water film runs on. On top of that sit the Riffle Cleat cleats, laid in a fanned pattern by the Riffle Strips. The riffles run in the direction of the shake and taper down toward the concentrate end, so heavy grains held behind a deep riffle near the feed can ride over the shallow end near the discharge into the Cleaning Zone, a smooth triangle where the film thins and the concentrate gets its final upgrade. Feed and water enter along the Edge Rail.

The asymmetric head motion

Separation depends on the deck shaking unevenly. The Head Motion Mechanism gives a slow forward stroke and a fast return. An Eccentric Shaft in the Head End Housing drives a Connecting Rod that pushes the deck gently forward, then a Toggle Block snaps it back: the carriage strikes the Knock-off Block against a Bumper Pad, and the Return Spring pulls it sharply home. On the slow forward stroke heavy grains are carried with the deck; on the fast return the deck slips out from under them, so over many strokes the heavy band creeps toward the concentrate corner while light grains, sitting higher in the water, do not follow. The Stroke Adjuster sets the throw from 10 to 30 mm, and the Head Motion Motor turns the shaft at the set frequency through a Head Reducer.

Wash water and film flow

The Wash Water System system lays a thin even sheet of water across the deck. Slurry enters the Feed Box at the head corner, and clean wash water runs the length of the upper edge in the Water Launder, overflowing onto the deck through the Weir Lip as a smooth film rather than jets. The film carries light grains down the slope to the tailings edge while leaving the heavies behind the riffles. Each Water Valve trims its section so the film does not run dry at one end or flood at the other, since an uneven film smears the band boundaries. The Feed Valve holds the slurry rate steady, because a surging feed shifts the whole pattern.

Slope and suspension

The deck shakes along one axis only and tilts about another. The Deck Suspension carries it on Rocker Support and Flexible Support Leg supports that allow the horizontal stroke and return the deck after the knock-off. The whole deck sits on a Deck Carriage that the head motion drives. Cross slope, the slope across the line of shake, is set by the Tilt Screw acting on the Slope Trunnion pivots, turned by the Tilt Handwheel. Slope from level to about 10 degrees controls how fast the film runs and so how sharply heavies and lights are split. The operator sets it by eye, watching the bands form, then trims water and stroke to sharpen them.

Product collection and drive

Along the lower discharge edge the Product Collection catches the fanned bands. The Concentrate Launder takes the heavy band that has travelled the deck, the Middlings Launder takes the mixed band for a second pass, and the Tailings Launder takes the washed-off gangue. A pair of Product Splitter blades let the operator cut sharp products by moving where one band ends. The eccentric is turned by the Drive Train, where a Drive Motor drives a Jackshaft through V-belt belts, and a Flywheel smooths the pulsing stroke load so shaft speed barely dips on the forward push.

Control

The Control System keeps the three variables, frequency, slope and water, where the operator set them. A Motor Starter switches and protects the motors, and a Variable Frequency Drive varies stroke frequency from 240 to 360 strokes per minute without changing pulleys. A small PLC Controller logs feed and water and trips the table on loss of wash water, since a deck running dry lets concentrate wash off into the tailings. A Water Flow Sensor watches the supply for that condition.

Variants and use

Shaking tables are a fine-particle gravity device, working from about 2 mm down to 20 microns at 0.5 to 2 t/h per deck. They recover gold, tin, tantalum, tungsten and chromite, and clean coal of fine pyrite. Because one deck has a low capacity, plants stack two or three decks on one head motion to save floor space, or run banks of tables in parallel. Smaller laboratory and pilot tables use the same geometry to check whether a feed will respond to gravity before a full plant is built. The table is slow but cheap to run, uses only water and gravity, and gives a clean visual readout of the separation, which keeps it in use where finely liberated heavy minerals must be recovered without chemicals.