Gravity Concentaration By Jigs Experiment Engineering Essay

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Jiging is one of the specific gravity separation methods.The method separate minerals of different specific gravity by their relative movement in response to gravity. Particle size is also important in jigging.If the feed is closely sized,it is easy to get good separation with narrow specific gravity range.Jigging is suitable for the sizes between 25 mm and 75 microns for minerals and suitable for 20 cm and 0.5 cm for coal.

2)THEORY:

Jig is an open tank filled with water that has screen at the top and spigot or hutch compartment at the bottom.Jig bed may have heavy coarse material(ragging material).

Jigs have screen,stroke length hutch compartment under the screen and ragging,a layer of heavy material.

There are two actions at work.First one is most important one that is the effect of hindered settling.When the slurry is subjected to several pulses before it exists the tailings weir of the jig,better separation will get.

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After repeated pulses,particles become stratified.Heavy materials are settle at the bottom and light particles settle at the top.The other action is the effect of the water.Upward flow of water separates particles by their specific gravity.

Some conditions present in jigging action are;

1)Therminal Velocity:Initially particles have an acceleration and increasing velocity.When equilibrium is achieved,particles reach their terminal velocity and they settle down at constant rate.

2)Free Settling:The sinking of particles in fluid.

3)Hindered Settling:The hindered settling conditions prevail when the proportion of solids in the pulp increases.The effect of particle crowding becomes more apparent and falling rate of particles begins to decrease.The system begins to behave as a heavy liquid whose density is that of the pulp rather than that of the carrier liquid.

Mechanisms;

1)Differential Initial Acceleration:The initial acceleration is dependent only on the densities of the solid and the fluid.It is necessaryu that short jigging cycle to separate small heavy particles to light particles.

2)Consolidation Trickling:In consolidation stage,where the large particles in the bed come close to each other leaving relatively large interstices filled with draining water running down as a result of the suction part of the strike.

Separation may be achieved over the screen or trough the screen in jigging.

The operation parameters of jigs are;

1)Dilution:It is the amount of water.High dilution is necessary to remove large quantity of materials.

2)Screen Aperture:It must be as large as possible,consistent with feed size to minimize resistance to flow.

3)Stroke and Frequency:Stroke is moving distance of the piston and it depends on particle size.Frequency is the number of stroke per time.

4)Feed Rate and Particle Size Range:Jigs have high unit capacity and can achieve good recovery in particle size under 150 pm.

3)OBJECTIVES:

-To observe the stratification process

-As a function of the value of concentration criteria,observing the rate of stratification

-To observe the effect of the ragging material

-To observe the effects of the operating variables

4)MATERIALS AND EQUIPMENT:

-multi compartment piston type laboratory jig steel balls as ragging material

-crushed heavy mineral with the size of -3mm +0,5mm

-crushed light material wit the size of -3mm +0,5mm

-chromite

5)PROCEDURE:

-prepare a mixture of heavy and light minerals

-fill the jig wit hutch water

-add the feed

-add steel balls as ragging material

-start the jig and observe the stratification

-empty the stratified material

-collect the light product

6)DISCUSSION:

a)Jigs: The methods operate by differences in specific gravity. Jigs rely on stratification in a bed of coal when the carrying water is pulsed. The shale tends to sin, and the cleaner coal rises. The basic jig, Baum Jig, is suitable for larger feed sizes. Although the Baum Jig can clean a wide range of coal sizes, it is most effective at 10-35 mm. A modification of the Baum Jig is the Batac Jig which is used for cleaning fine coals. The coal is stratified by bubbling air directly through the coal-water-refuse mixture in this cleaning unit.

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For intermediate sizes the same principles are applied, although the pulsing may be from the side or from under the bed. In addition, a bed or hard dense mineral is used to enhance the stratification and prevent remixing. The mineral is usually feldspar, consisting of lumps of silicates of about 60 mm size.

Jigs offer cost effective technology with a clean coal yield of 75-85% at about 34% ash content. The jigs are used more frequently than dense-medium vessels because of their larger capacities and cheaper costs.

b)Baum jigs and Batac jigs:

i)Baum jigs: A baum jig with a screen deck comprising, in combination, a number of water cisterns each having a feed end and a discharge end and arranged in a series-paralleled system withsymmetrical air chambers opened at the bottom and located beneath the screen deck transversely to the axis of the jig, said symmetrical air chambers comprising plates as sections of a cylindrical surface, the symmetry axes of said air chambers beingdeflected from the perpendicular by an angle of 5° to 15° toward said feed end, said chambers having asymmetric guide vanes displaceable in a horizontal direction.

ii)Batac jigs: There is one decisive difference between an BATAC jig and a Baum jig: The water current is not generated in an air chambers are intermittently supplied with compressed air by an electronically controlled valve or flap system (pulse generator). That air is intermittently discharged from the system (at atmospheric pressure) after completion of the upward stroke. Motion is imparted to the water inside the jig as a function of the pressure generated inside the air chambers. Moreover make-up water is added at the lowest location of every jigging chamber to intensify the upward current and to dampen the downward current. The feed is stratified according to its density by the pulsating motion of the water: when preparing coal, e.g., in refuse, middlings and clean coal.

The heavy fraction of the stratified raw material is sensed by floats in accordance with the product qualities required. The system provides for controlled withdrawal of the heavy fraction over a discharge device. Jigging of great width are equipped with independently operating discharge devices fitted with separate sensor and hydraulic units. This configuration ensures optimal product qualities over the entire jig width even in case of non-uniform material distribution.

The parts of the batac jig:

inlet

refuse outlet

refuse collecting hopper

middlings outlet

middlings collecting hopper

discharge devices

clean coal overflow

air chambers

make-up water

air-distributing tabes

air-distributing tank

waste-air collecting tank

waste-air tubes with silencers

valve control system

Operating parameters of jigs: The monitoring and control of jig separators is effected by monitoring the time variation within a jig cycle of at least one operating parameter of the jig, and manipulating the operating parameter(s) to produce the sought after form of the time variation within the jig cycle. Operating parameters include bed voidage, water level, particle velocity in the bed and water or air pressure.

c)An example of flow sheet:

In the flow sheet below, you can see the details of this jigging process as it exists on a Superior mill floor. Copper rock from the stamp enters the classifier and is directed to one of four jig sieves. These jigs work in tandem and both copper and tailings are removed during the process (the T and C in the diagram). The middlings are then sent on to a distributor box, which sends the tailings off to one of four series of refining jigs. More copper and tailings are removed as the middlings are sent on to the wash floor.

7)RESULTS:

In this experiment, we have learned the working principle of the jig, the effect of the ragging layer, the stratification process, the effect of operating parameters of jigs. Jig separate the minerals by the effect of specific gravity, and some other forces such as hindered settlings of minerals etc. and the minerals has a layer (stratification) by means of an upward water and the help of the ragging layer. In addition, there are some parameters which effect the separation. They are design parameters such as stroke and frequency and operating parameters such as feed rate and particle size range. Furthermore, if we supply a narrow size range of the minerals to be separated, we can increase the effect of specific gravity and have a good separation.

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