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An Introduction to Jigs

Using jigs to concentrate coal and minerals has been around for a long time, the use of jigs for concentrating ores was reported in De Re Metallic by Agricola published in 1556.  And he referenced earlier work going back to Egypt almost 3000 years ago.  Early ore jigs used a basket, loaded with ore, that was moved up and down (jigged) in a tank of water.  This jigging allowed the particles to become rearranged in layers of increasing density from bottom to top.  The same principle is used in modern jigs to stratify and separate the particles of differing density.  For more information on how jigs work see SDM an Introduction to the Principles of Jigging.

 Jig circuits can be one of the simplest mineral processing circuits, primarily as jigs are rather forgiving in operation.  Jigs come in different sizes and shapes, but in general, a jig plant is essentially a materials handling system that is designed to supply ore in a specified physical state to the jigs and to carry away the products. 

 TYPICAL CIRCUIT

The most basis example is a placer jig circuit (Figure 1).  It consists of a feed hopper and conveyor (sometimes not included), a trommel to remove large oversize rocks, the jig, a water circuit, and a tails handling method.   

Figure 1: Basic Placer Jig Circuit

The other is a coal cleaning circuit (Figure 2) which consists of a Baum jig usually fed directly from plant feed, producing a final reject from the first compartments, a recycled middlings (usually crushed) from the second, and a clean coal.  The clean coal is often screened for fines processing, due to inefficiency of the primary jig cleaning fines.

 

Figure 2: Coal (Baum Jig) Circuit

 

Jigs use a large amount of water so reclamation of water with recycling is standard practice. Clean water is not necessary as clay and fully suspended slimes do not appreciably affect results up to an apparent density of 1.05. However, provision must be made to clarify enough water to stay below this level. In some cases chemical additions may help.

 

EQUIPMENT DESCRIPTION

The fundamental principles jigging are essentially the same for all jigs, the basic differences between the various types of jigs are a matter of practical engineering to optimize the operating performance, materials handling, maintenance and control.

 

The earliest jig was a moving bed jig (see De Re Metallic by Agricola). The screen containing the bed is jigged up and down in water to create the liquid pulse. There are some significant advantages to moving bed jigs both in performance and water consumption.  But since they are not commonly found, this discussion will be returned to in another paper.  Currently, the majority of jigs found are of the fixed bed type in which the liquid pulse passes up and down through the jig bed which is retained on a stationary screen. The different types of fixed bed jigs are given shown in Figure 4.  

 Figure 4: Fixed Bed Jig Types 

Type	Pulsating Mechanism	Stroke Modification
Baum	Air	Air discharge control
Richards	Water	Water valve control
Harz	Mechanical Piston	Differential piston action
Denver	Mechanical Diaphragm	Water valve
Bendelari	Mechanical Diaphragm (internal)	none
Yuba	Mechanical Diaphragm (side)	Hydraulic or mechanical
Pan-American	Moving Hutch	Mechanical
Russian MOBK	Pneumatic piston	Air discharge control

 

For coal cleaning, the Baum (Figure 5) and Batac jigs are the most common type.  For other minerals, the Pan-American (mechanical or diaphragm) jig is the most commonly used type. This dif­ference has more to do with through put capacity, especially float capacity.  Baum and Batac jigs of 1500 tons/hour capacity exist.  A small Baum jig would have a capacity of 50 tons/hour.  A jig of 100 tons/hour capacity is large for a mineral jig. Large mineral jigs, and small Baum jigs can be found.  But the above pertains to most common capacities.

Figure 5: Typical Baum Jig

Batac jigs are Baum style jigs, with the air chest under the deck screen (usually on the divider wall between cells).  Other than this they are similar in operation and capacity to Baum style with improved performance reported for fine sizes.  Having the air chests under the screen requires less floor space.  They also tend to have improved instrumentation giving better control.  Sizing is similar to a Baum jig.

 Baum style jigs usually have two compartments, a primary and a secondary (figure 5).  Coal is normally fed to the primary where the main reject is produced.  Coal from the primary feeds the secondary which produces the clean coal.  The secondary reject may be a second or middlings product (market depending), recleaned in another jig, combined with the primary reject, or recycled (partly) to the feed. 

 A common way of describing a Baum jig is by giving the width, number of primary cells, and then the number of secondary cells.  A 734 jig would be seven (7) foot wide, with three (3) primary, and four (4) secondary cells.  For most jigs the normal cell is 3 foot wide.  Jig bed area would be calculated as follows:

           Sq. ft. = No. of cells x jig width x 3 or for the 734 jig;

              7 foot wide jig = 7 x 7 x 3 =  147 sq. ft²

 A few three compartment Baum jigs have been used, adding a tertiary stage.  This would be used for a coal having a large amount of near gravity material.

 The most common mineral jig in use today is the Pan-American style, although some Yuba and Denver jigs can be found.  

 Circular jigs tend to fall into two groupings, single hutch round jigs and ones made of individual cells like segments in an orange.  In both cases improved performance is gained by allowing the bed to spread, allowing finer particles to separate better.  But at a space penalty and in more complicated feed and concentrate systems.   General sizing is similar to other mineral jigs.

Figure 6: Typical Pan-American Mineral Jig

 The typical Pan-American (often just Pan-Am) jig as shown in Figure 6, is normally built as a duplex jig (a pair of balanced jig cells). Each cell is consists of an upper hutch (usually rectangular), a lower hutch (usually conical), joined by an annular diaphragm of flexible rubber to allow up-and down movement of the lower hutch. 

 The typical Pan-Am consists of the following:

• two cells each 42” x 42” (1 m x 1 m);
• common drive (energy savings);
• amount of ragging – typically 425lbs (193 kilos) per cell;
• type of ragging – typically 3/16 inch (4.75mm) steel shot;
• stroke length – ¾ inch to 1½ inch (19 to 38mm);
• stroke frequency – 120 to 200 cycles per minute.

 Pan-Am duplex jigs are popular in many regions, and can be seen in action in Alaska, Yukon, South America, and Africa, much of this is due to their inherent simplicity and relative ease of maintenance.

EQUIPMENT SIZING

Jigs are sized on the feed tonnage per unit of jig area.  Different types of feed will require different sizing criteria.  Specifically coal jigs have different sizing criteria than mineral jigs.

 Coal Jigs (Baum & Batac) capacity depends on the amount of fine feed.  The greater the amount of fines, the lower the basic jig capacity.  If the amount of fines is above 30% by weight, the fines may size the jig.  While a portion of the fines may be removed by classification ahead of the jig, some fines are needed.  If a classified (screened) feed is used capacity will decrease or greater amounts of water will be needed.   In this case fines being the material that is ¼ of the average particle size.  Part of this is due to normally larger particle size being feed in coal.  Feed sizes of over 6” top size are common.   Since a large portion of the work in separation is done processing marterial near the gravity of separation, for coal feeds that have a high amounts of material near the gravity of separation, the jigs capacity maybe limited by its refuse handling capacity.

 For coal jigs, normal water use is 7 to 8 gal/min of water per ton/hour of dry solids.  This water is split 30% to the feed, and the remainder to cells.  A more precise way of cal­culating water requirements is in gal/min per square foot of jig area allowing for the dilution of the fines.  Fine particles are needed to create a semi- suspensoid  (heavy medium).   Typical coal jig capacities are shown in Table 1 and Figure 7.

Table 1 Baum Jig Capacity

Baum Jig Capacity
T/Hr per Foot2 Jig Area
% < 6.35 mm	Jig Capacity
(-1/4")	(T/Hr/Foot2)
< 25 %	4 - 5
25% - 30%	3 - 4
>30 %	2 - 3
100%	1 - 1.5

 

Figure 7: Baum Jig Capacity

 Baum and some Batac jigs use bucket elevators to drain and discharge the refuse. The physical ability to ex­tract the refuse may be the sizing criteria. A check is also made of the sink (refuse) handling capacity (figure 8).

 

Figure 8 Baum Jig Elevator Capacity

Normal water use is 7 to 8 gal/min of water per ton/hour of dry solids.  This water is split 30% to the feed, and the remainder to cells.  A more precise way of cal­culating water requirements is in gal/min per square foot of jig area allowing for the dilution of the fines.  Fine coal particles (minus 6.35 mm (1/4")) are needed to create a semi- suspensoid  (heavy medium).  Table 2 gives general requirements for Baum jigs.

Table 2: Baum Jig Water Requirements

Baum Jig Water Requirements
G/min per Foot2 Jig Area
% - 6.35 mm	Maximum	Minimum	Average
< 25%	45	30	37.5
25 to 30 %	30	17.5	23.5
> 30%	15	5	10
Average conditions  = 32.7 g/min
Average cell is 3' x 8' or 24 ft2 = 785 G/m/cell

 For mineral jigs, capacity is heavily dependent on the average specific gravity of the feed.  This determines the volume of material (dry basis) that is fed to the jig.  Jig design also affects capacity, as circular jig have a higher unit capacity then rectangular jigs.  This comes from the spreading of jig be towards the overflow weir, allowing for a less dense bed improving fines separation.  For any particular operation there is a "best" combination of the jig adjustment factors. Their selection is often is based on experience and operator preference, and is open to much debate as to what is "best".  The following are some of the major adjustment factors:

 ·         Stroke Speed and length. Balancing the stroke speed and length are two important aspects, and can lead to good or poor performance.  Jigs have a natural period of motion which results in an optimum speed for any given type and size of jig.  The Material characteristics also impact this.

 ·         Water Flow. The jig stroke is modulated by the water upflow.  Too much hutch water can cause losing fine values to the tails.  Too little hutch water can can cause the reporting of tails to the concentrate.

 ·         Ragging. Determining the proper type and size of ragging is important in the final specific gravity cut point.  The tails will in general be lighter than the ragging, while the concentrate will be heavier.  Ragging size also limits the size of the coarse material that will pass.

 ·         Jig Slope. The bed must be sloped across large jig machines to provide bed movement. Jig design must allow for adjustment of this slope in the installation.

 ·         Jig area.  The area of the jig is what determines the feed capacity.

 Table 3: General Mineral Jig Capacity

	Mineral Jig Capacity
	T/Hr per Foot2 Jig Area
Average Feed	Low		High
Sp Gr	End	Average	End
2.35	1.5	1.5	1.6
2.50	1.6	1.6	1.7
2.75	1.7	1.8	1.9
3.00	1.9	2.0	2.0
3.50	2.2	2.3	2.4
Jig configuration and particle size will have some impact
but an average of 1.8 Tons/hr/ft2 of jig area is common

 Figure 9: General Mineral Jig Capacity

 EXAMPLE

The following is an example of sizing and selecting a coal (Baum style) jig.  It is included for reference only.   In actual practice many different factors can cause the specific selection to change.

 Conditions: 

500 ton/hour of raw coal

6" top size

25% minus 1/4"

85% (at 1.65 Sp.Gr.) reporting to clean coal.

 With 25 % minus 1/4", from table 1 and figure 7, the average design capacity would be 3.5 tons/hour/foot².  500 tons/hour of raw coal requires:

                      500 T/hr        =   142.8 Ft²  or 143 Ft²

                      3.5 T/Hr/Ft²

 From above, a 734 jig (7 foot wide, 7 cells) has a jig area of 147 Ft².

 Checking refuse handling capacity gives:

                          100 - 85  x 500 T/Hr   = 75 T/hr

                                    100

 Of this, normally 75 to 80 % reports to the primary elevator, or design for 60 T/hr in the primary.  From figure 8, this would require a 22 inch wide elevator as a min­imum.

 Water requirements based on size (Table 2) are 23 G/min/ Ft² or:

                         147 Ft2 x 23 G/min/ Ft2 = 3381 g/min

 For a feed rate of 500 T/hr the average water requirement would be 3500 to 4000 g/min.  As the amount of minus 1/4" material is low, using 3400 G/min is probably adequate as a starting point.  Of this, 30 % (1020 G/min) would be added to the feed and the remainder to the cells (340 g/min each).

 In general a mineral jig is sized in a similar manner.

Conditions: 

50 ton/hour of placer feed – to trommel

1/2" top size

Average feed density – 2.75 gm/cc (~2.75 Sp. Gr)

Concentrate - > 5 gm/cc

 Assuming some reject of material by the trammel (~10%), would give a projected jig feed of 45 tons/hour.  From Table 3 and figure 9 the average capacity would be 1.8 tons/hour/foot².  45 tons/hour of feed requires:

                     50 T/hr        =   25 Ft²

                    1.8 T/Hr/Ft²

 A standard duplex Pan-Am jig (42” x 42” x 2)  has ~24.5 Ft², of area and would be a good choice,