Mineral Jig Concentrator

Overview

A mineral jig is a gravity concentrator that sorts a feed of mixed particles by density. It pulses water up and down through a bed of mineral resting on a screen. Heavy particles settle through the bed and report to the concentrate under the screen, while the lighter gangue stays near the top and overflows as tailings. The jig works on density alone, so it suits minerals that are much heavier than the rock around them: gold, cassiterite (tin), tungsten, tantalite, chromite, garnet, and the heavy fraction in iron and coal washing. It uses no reagents, only water, which is part of why it stays in use long after froth flotation took over much of mineral processing.

The machine is built from a few subsystems on one frame. The Jig Tank and Hutch holds the water and the cells, the Jig Bed and Screen Deck is the screen and the heavy media that sit on it, the Diaphragm Pulsion Mechanism makes the water pulse, the Hutch Water System adds makeup water under the screen, the Concentrate Discharge draws the concentrate off, and the Drive Unit turns the pulsion mechanism. The Control and Instrumentation holds the pulse, water and draw at steady settings.

The jigging principle

Feed slurry runs in over the Feed Launder and spreads across the Punched Screen Plate. Under each screen is a Hutch Chamber, a chamber full of water. The pulsion mechanism drives that water up through the screen and then lets it fall back. On the up stroke the whole bed lifts and opens up, so the particles are briefly suspended and free to move. As the water slows and reverses, the particles fall back at a rate set by their size and density. Heavy particles fall faster and reach the screen first; light ones settle on top. After many cycles the bed is stratified, heavy at the bottom and light at the top. The light layer is washed off the Tailings Overflow Weir as tailings, and the heavy layer at the screen is drawn off as concentrate. The down stroke matters as much as the up stroke. As the water drops back through the bed it sucks fine heavy grains down through the gaps, an action called consolidation trickling, which is how a jig recovers particles smaller than the screen holes.

The bed and ragging

A jig does not run on bare screen. A layer of Ragging Media sits on the Punched Screen Plate, a charge of dense coarse media such as steel shot or hematite, held in place by a Ragging Retainer. The ragging is the working part of the bed. The pulse dilates it on the up stroke, and the size and density of the ragging set how easily mineral passes through to the hutch. Coarse, heavy ragging passes only the densest mineral and gives a clean, high-grade concentrate at lower recovery; finer, lighter ragging passes more and lifts recovery at the cost of grade. The holes in the screen are sized to retain the ragging while letting the fine concentrate fall into the Hutch Chamber below. The Screen Frame holds the deck over the hutch and is bolted down so the pulse does not lift it.

The pulsion mechanism

The pulse is made by the Diaphragm Pulsion Mechanism, a diaphragm drive. The Drive Unit turns the Eccentric Shaft, whose offset throw works the Connecting Rod up and down. The rod flexes the Rubber Diaphragm, a reinforced rubber membrane sealing the bottom of the pulse chamber, through the Diaphragm Backing Plate. As the diaphragm rises it pushes water up through the bed; as it falls it draws water back. The Flywheel on the shaft smooths the load so the motor turns at a steady speed through the stroke. Two settings govern the separation. The Stroke Adjustment sets the stroke length, how far the diaphragm moves and so how far the bed lifts, in the range 5-50 mm. The drive speed sets the pulse frequency, 100-300 cycles per minute. Coarse feed wants a long, slow stroke; fine feed wants a short, fast one. The diaphragm is the main wear item of the drive and is changed on a schedule, since a split diaphragm stops the pulse.

Water and discharge

The pulse alone does not move enough water in one direction. The Hutch Water System adds makeup water under the screen through the Water Manifold and a Hutch Water Valve on each hutch, fed by the Makeup Water Pump. This hutch water lengthens and strengthens the up pulse and gives a net upward flow that keeps the bed open and carries the tailings across to the weir. The Flow Meter lets the operator balance the water cell to cell. Concentrate is drawn off through the Concentrate Discharge. Each Hutch Chamber feeds a Spigot Discharge Valve whose Pinch Sleeve meters the draw without losing the water seal. On a controlled machine a Density-Controlled Spigot reads the bed with a Density Float and a Pressure Sensor and opens the Modulating Valve as heavy mineral builds up, so the grade holds steady when the feed changes. The drawn-off concentrate runs down the Concentrate Launder to the Concentrate Collection Box.

Drive and frame

The Drive Unit is a Drive Motor through a Gear Reducer and a set of Drive Belt vee belts. The reducer brings the motor speed down to the slow turn the eccentric needs for the pulse frequency. A Variable Frequency Drive on the motor sets that speed, which is how the operator trims the pulse frequency without changing pulleys. Everything sits on the Base Frame, carried on a Support Leg at each corner. A Anti-Vibration Mount and a Coil Spring under each foot keep the pulse reaction out of the floor, which matters because the jig shakes a heavy water mass twice a second.

Control, variants and use

The Control and Instrumentation ties the parts together. The PLC Controller times the pulse, sequences the hutch water and steps the spigot valves from the Bed Density Sensor signal, with water pressure, bed level and stroke read by the Process Sensor Set. Jigs vary by how the pulse is made. The diaphragm jig here is the common modern type. A plunger jig uses a piston instead of a diaphragm; an air-pulsed (Baum or Batac) jig uses compressed air on the water and is standard for washing coal at high tonnage. By bed type, a fixed-sieve jig keeps the screen still and pulses the water, as here, while a moving-sieve jig shakes the screen itself. Jigs are used to recover heavy minerals from alluvial and hard-rock ores, to wash coal, to clean scrap metal, and to pre-concentrate ahead of finer processing. The appeal is a simple machine that separates by density with water and no chemicals, at a low cost per tonne.