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Gravity concentration methods separate minerals of different specific gravity. They are used to treat a great variaty of materials [ranging from Au ( sp. gr. 19.3 ) to coal ( sp. gr. 1.3 ) ]. Gravity concentration methods remained, however the main concentrating methods for iron , tungsten, tin ores and coal. This methods are usually preferred to flotation due to its low cost . Minerals liberated at sizes above flotation range may be concentrated even more economically using gravity methods (also cause efficient dewatering due to decreased surface area.). The main principles of appliying this method is different size and shape of minerals having different specific gravity. This method can be applied at the range of 7.5 – 1.3. And, the advantages of this method are its simplicity, having high capacity and low cost. In order to apply gravity concentration method concentration criteria ( cc ) shoud be at optimum range.
CC = dh – df
dl – df
CC >2.5 ( 74 micron )
CC <1.25 HMS depending on the liberation size it can be
CC = 2.5-1.75 ( 150 micron ) applied easly
CC =1.75 -1.50 ( 1.7 micron)
CC = 1.5 – 1.25 ( 6mm )
JIGGING:
Jigging is a kind of gravity concentration method. This method seperates minerals of different specific gravity by their relative movement in response to gravity and ore or more other forces the latter often being the resistance to motion offered by a viscous fluid, such as water or air. The jig is normally used to concentrate relatively coarse material. Good separation is possible if the feed is closely sized (e.g. 3 – 10) and if the sp. gr. difference is large. Many large jig circuits are still operated in the coal, cassiteriite, tungsten, gold, barytes and iron-ore industries. Jigs have a relatively high unit capacity and can achieve good recovery of values down to 150 µm and acceptable recoveries often down to 75 µm. (High proportions of fines interfere with performance.).
Jigging is the stratification of minerals of different sp. gr. The separation is accomplished in a bed which is rendered fluid by a pulsating current of water so as to produce stratification. The aim is to dilate the bed of material being treated and to control the dilation so that the heavier, small particles penetrate the interstices of the bed and the larger high sp gr particles fall under a condition probably similar to hindered settling. On the pulsion stroke, the bed is normally lifted as a mass , the bottom particles falling first until the whole bed is loosened. On the suction stroke, it then closes slowly again and this is repeated at every stroke; the frequency varying between 55 – 330 cycle / min. The fine particles tend to pass through the interstices after the large ones have become immobile. The motion can be obtained either by using a fixed sieve jig and pulsating the water, or by employing a moving sieve in a simple hand- jig.
Jigs can be divided into two groups. One of them is Mineral Jigs ( 16mm – 100 micron ) This jigs is seperated into two groups: Harz and Denuer. And, another one is Coal Jigs
( 25mm – 150 mm). This jigs is also seperated into two groups. These are Baum and Batac.
Mechanism:
Seperation in jigs is achieved by using two mechanizm : over the screen and through the screen.
Differential initial acceleration: particles are subjected to pulp at the beginning so they accelerate according to their specific gravities but not their size.
Consolidation trickling: At the end of a pulsing stroke in the 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 stroke.
In the jig , the movement of the piston is a harmonic waveform.
The speed of flow through bed during jig cycle is sine – curve.
Operation Parameters of Jigs:
1 – Dilution
2 – Screen Aperture
3- Stroke and Frequency
4 – Feed Rate and Particle Size Range
Some conditions that take place in the jigging operation
Terminal velocity: As the object keeps on accelerating downwards, the drag produced is increased. At a particular speed, the drag force produced will be equal to the downward force, mostly the weight (mg), of the object. Eventually, it plummets at a constant speed called terminal velocity. Terminal velocity varies directly with the ratio of drag to mass. More drag means slower terminal velocity. Increased mass means higher terminal velocity. An object moving downwards at greater than terminal velocity (for example because it was affected by a force downward or it fell from a thinner part of the atmosphere or it changed shape) will slow until it reaches terminal velocity.
Free settling: An object that sinks in a fluid because of just gravity is said to be free falling.
Hindered settling: When the proportion of solids in the pulp increases hindered settling occurs. In this situation particles move slower under the affect of crowded pulp.
Objectives
To demonstrate the effect of the ragging layer
To see the effect of operating parameters of jıgs
To demonstrate the stratification process
To observe the difference in the rate of stratification as a function of the value of the concentratin criterion
Experimental
Materials:
Quartz
Chromite
Usage areas of Quartz in Industry:
Quartz crystals have piezoelectric properties, that is they develop an electric potential upon the application of mechanical stress. An early use of this property of quartz crystals was in phonograph pickups. One of the most common piezoelectric uses of quartz today is as a crystal oscillator. The quartz clock is a familiar device using the mineral. The resonant frequency of a quartz crystal oscillator is changed by mechanically loading it, and this principle is used for very accurate measurements of very small mass changes in the quartz crystal microbalance and in thin-film thickness monitors. Quartz is used in industry for following purposes;
At physical and chemical researches
For fiber which is used for high temperature isolation and acid filtration
For tubes at furnaces which are heated with gas and electric , including vacuum furnaces
To produce tubes and arms for casting
For optical lenses , prisms and disks
Usage areas of Chromite in Industry:
The following uses for chromium are gathered from a number of sources as well as from anecdotal comments. I’d be delighted to receive corrections as well as additional referenced uses (please use the feedback mechanism to add uses).
used to harden steel, to manufacture stainless steel, and to form alloys
used in plating to produce a hard, beautiful surface and to prevent corrosion.
used to give glass an emerald green colour. It is responsible for the green colour of emeralds and the red colour of rubies
wide use as a catalyst
dichromates such as K2Cr2O7 are oxidising agents and are used in quantitative analysis and also in tanning leather
lead chromate as chrome yellow is a pigment
compounds are used in the textile industry as mordants
used by the aircraft and other industries for anodising aluminium
the refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stable crystalline structure
tanning leather
Equipment:
Multi compartment piston type laboratory jig with a screen of 3 mm aperture, and 200 gr of stainless steel balls of 4 mm diameter as ragging material
Crushed heavy mineral in the size of – 3mm + 5mm (Chromite)
Crushed light mineral in the size of – 3mm + 5mm (Quartz)
Procedure:
By using of the batch laboratory jig;
a mixture of one heavy mineral ,one light mineral a bed of stainless steel balls as ragging material was ready on only one of the jig screen.
Fill up the jig with hutch water
Start the jig and observe the rate of stratification of mixture components
Collect the light product that lows over the discharge weir.
4.Results and Discussion
A Baum Jig for gravitational enrichment of minerals, particularly hard co in which a screen deck is transversely located to the axis of the jig. Air chambers are made of plates as cut-outs of a cylindrical surface. The axes of symmetry are deflected from the perpendicular by an angle of 5° to 15° in the direction of the mineral to be enriched. The chambers are provided with asymmetric guide vanes adjustable in the horizontal direction. The side walls of a water cistern possess at the height of the screen decks, horizontal off-sets outside the cistern. Spring-loaded protection plates are provided in the off-set supports. The lower edge of one wall of each chamber is shaped as a conduit of back water provided from the bottom with a deflectable arched diaphragm.
Batac Jig System used in Iron Ore Application
∞system in an iron ore application in South Africa, at the Assmang Beeshoek mine in Northern Cape.
The two air-pulsed jigs, one of which is utilised to process lumpy ore while the other processes fines material, operate on the well-proven, simple principle of employing an underlying air pulse to lift and segregate material according to density.
Although the technology has a proven track record internationally, it is considered new in South Africa. The project has drawn much interest from industry as the jigs are reported to be producing above expectations, with 50,000t of ore processed while ramping up in May.
The overall project involved the installation of an additional crushing, screening and jigging plant with a water reticulation system. Humboldt Wedag SA was the turnkey contractor for the jig plant and, with its associates, supplied a turnkey processing plant incorporating the two jigs, and ancillary equipment including screens, bucket elevators and material handling systems.
5.Conclusion
Jigs are easy operated devices with high efficiency and applicability. Many parameters should be considered in use of jigs to get the best result. It has some advantages over other concentration devices. First, it is simple to operate. Second, it requires less labour and floor space. Moreover, it has a low maintenance cost. Furthermore its energy consumption is considerably low as it works only with a pump to pulsate water. Jigs can be used to achieve a good recovery in particle size below 150 µm. It can be said that although its construction is very simple, it has many actions at work affect seperation of minerals of different gravities. And also, its mechanism is very simple to seperate the minerals with different specific gravities.
