jigging, Mineral Processing, iron and steel making.pptx

Jigging, Metallurgical and Materials Engineering

1. Jigging Mineral processing

2. Jigging is one of the oldest methods of gravity concentration, In the jig the separation of minerals of different specific gravity is accomplished in a bed which is rendered fluid by a pulsating current of water so as to produce stratification. The jig is normally used to concentrate relatively coarse material and, if the feed is fairly closed sized (e.g. 3-10mm), it is not difficult to achieve good separation of a fairly narrow specific gravity range in minerals in the feed (e.g. fluorite, sp. gr. 3.2, from quartz, sp. gr. 2.7). When the specific gravity difference is large, good concentration is possible with a wider size range. Ex: Many large jig circuits are still operated in the coal, cassiterite, tungsten, gold, and iron-ore industries.

3. Aim of jigging To dilate the bed of material being treated and to control the dilation so that the heavier, smaller particles penetrate the interstices of the bed and the larger high specific gravity particles fall under a condition probably similar to hindered settling.

4. Conventional jig

5. Jigging: Jiggs are commonly used to clean coal but are also used in heavy mineral separations including gold. When treating coal, the light fraction is the concentrate and in the mineral industry the heavy fraction is the concentrate. The jig is commonly an open tank filled with water, with a horizontal screen near the top, Some early jigs were designed where the screens surface, in the form of basket, moved up and down in a barrel or tank of water, hence producing the vertical flow of the fluid through the bed of particles. Some movable screen jigs are still designed today through most modern jigs employ a stationary screen and pulse the water through it.

6. The difference between the various types of jigs available related to the method used to generate the pulsation and the manner in which the heavy fraction is removed from the jigs. The screen is there to support the bed of the particles and the area underneath the screen is called the hutch. The tank is usually divided into two main sections: one containing the support screen with the bed of ore and the another section which generates the fluid pulse. The heavy discharge from the jig may be either through the screen or over the screen.

7. In jigging through the screen, all particles in the feed are smaller than the screen aperture and thus have the potential to drop through the screen and collect in the hutch. To stop the light fraction falling through the screen, a false support is provided in the form of a layer of coarse heavy particles ragging which then contacting the screen surface pack down to effectively close off the screen apertures to the feed particles. Example of ragging material is Feldspar for coal jigs. Large jigs are divided into separate compartments with different operating condition for each compartment, such as roughing and scavenging and cleaning duties etc.

8. For feed particles larger than the apertures of the supporting screen, jigging over the screen may be practiced, and the heavy product grade is partly controlled by the thickness of the bottom layer which in turn is controlled by the rate of withdrawal through the heaviest discharge port. Gates are operated to allow the heavy fraction to drop into a bucket elevator for removal. Positioning of the gate opening is controlled by the location of the boundary between light and heavy layers and this is determined by a weighted float positioned in the bed or monitoring the pressure fluctuations in the pulsating water.

9. Types of jig Essentially the jig is an open tank filled with water, with a horizontal jig screen at the top, and provided with a spigot in the bottom, or hutch compartment, for concentrate removal. The jig bed consists of a layer of coarse, heavy particles, or ragging, placed on the jig screen on to which the slurry is fed. The feed flows across the ragging and the separation takes place in the jig bed so that grains with a high specific gravity penetrate through the ragging and screen to be drawn off as a concentrate, while the light grains are carried away by the cross-flow to be discarded as tailings. The harmonic motion produced by the eccentric drive is supplemented by a large amount of continuously supplied hutch water, which enhances the upward and diminishes the downward velocity of the water

10. Harz jig One of the oldest types of jig is the Harz (Figure 10.10) in which the plunger moves up and down vertically in a separate compartment. Up to four successive compartments are placed in series in the hutch. A high-grade concentrate is produced in the first compartment, successively lower grades being produced in the other compartments, tailings overflowing the final compartment. If the feed particles are larger than the apertures of the screen, jigging "over the screen" is used, and the concentrate grade is partly governed by the thickness of the bottom layer, determined by the rate of withdrawal through the concentrate discharge port.

11. Denver mineral jig

12. Coal jig (used in coal industry) Jigs are widely used coal-cleaning devices, and are preferred to the more expensive dense medium process when the coal has relatively little middlings, or "near-gravity" material, like: British coals. No feed preparation is required, and for coals which are easily washed, i.e. those consisting predominantly of liberated coal and denser rock particles. Two types of air-pulsated jig – i.e. Baum jig and Batac jig Air under pressure is forced into a large air chamber on one side of the jig vessel causing pulsations and suction to the jig water, which in turn causes pulsations and suction through the screen plates upon which the raw coal is fed, thus causing stratification.

13. Coal jigs Suitable for coarse load, Feed size range : max 175-200mm min 40-60 mm Air pulsation type Two or three product (light materials, middling and heavy densed type) Automatic discharge modular design, bed area and elevator designed to suit duty. Designed to handle high portions of sinks compared to the mineral jig.

14. Principle of operation and fundamentals Stratification in a bed of particles results from the repeated pulsation of a current of fluid up through the bed. The particles in the bed are expanded so that when pulsation ceases, the particles are allowed to consolidate under the influence of gravity. The expansion and contraction of the bed is repeated in a cyclic operation until the heavy and light particles have stratified according to their specific gravity. Frequency of pulsation usually varies from 50-300 cycle per min.

15. Idealised jigging particle distribution over time

16. As a particle just start to moves from rest, the particle velocity is small and hence the drag force acting on the particle is negligible since the drag force increases with particle velocity relative to the fluid. That is the initials acceleration of the particles depend only on specific gravity of the solid and fluid and is independent of the particle size. Once the particle reach an appreciable velocity the fluid drag force becomes significant and it opposes the particle further acceleration to the extent that eventually the particle acceleration drops to zero and a constant terminal velocity is reached which will depend on the particle diameter as well as density.