MINERAL JIGS

MINERAL JIGS

Mineral jigs or gravity concentrators were designed to allow for separation of materials with different densities. This process is accomplished by flowing a stream of liquid-suspended material over a screen and subjecting the screen to a vertical hydraulic pulsation. This pulsation momentarily expands or dilates the screen bed and allows the heavier materials to work toward the bottom. Heavier material, or concentrate, that is finer than the screen openings will gradually work through the beds and the retention screen into the hutch, or lower compartment. That material, the concentrate, is continuously discharged from this compartment or hutch through a spigot. If the concentrate is coarser than the screen, it will work down to the top of the shot bed, and can be withdrawn either continuously or intermittently. The lighter material, or tailing, will be rejected over the end of the jig.

The mineral jig has been widely used in recovering valuable heavy minerals such as gold, platinum, tin, tungsten and lower density minerals, gemstones such as diamond and sapphire from alluvial or placer deposits. Jigs have also been widely used in hard rock mill applications for recovering of coarse heavy minerals liberated in open or closed grinding circuits. Base metals, iron, manganese ores and barite have also been recovered using jigs. Where the difference between the density of the mineral being sought and the gangue is large, as in the case of gold or platinum, the “Placer Jig” achieves very efficient recoveries even of minus 50 mesh particles.

The mineral jig has certain advantages in placer and hard rock mill flow sheets. In gold recovery the jigs produce highly concentrated products which can be easily upgraded by methods such as barrel amalgamation, treating across shaking tables or processing through centrifugal concentrators. In other placer operations the heavy minerals being sought are recovered efficiently and cheaply with similar high ratios of concentration. In iron, manganese and base metal treatment flow sheets, the jigs are operated to produce marketable grades of concentrate; or, as pre-concentration devices, to reject barren gangue prior to the ore entering the fine grinding section of the mill flow sheet.

The construction of the mineral jig results in maximum utilization of floor area and minimum head room requirements, permitting greater capacity per unit of operating floor area than, for example, shaking tables or other devices. The design of the machine contributes to minimum operating and maintenance costs and negligible parts replacement expense.

MECHANISM OF OPERATION OF MINERAL JIG

Jig concentrators are devices used mainly in the mining industry for mineral processing, to separate particles within the ore body, based on their specific gravity (relative density).

The particles would usually be of a similar size, often crushed and screened prior to being fed over the jig bed. There are many variations in design; however the basic principles are constant: The particles are introduced to the jig bed (usually a screen) where they are thrust upward by a pulsing water column or body, resulting in the particles being suspended within the water. As the pulse dissipates, the water level returns to its lower starting position and the particles once again settle on the jig bed. As the particles are exposed to gravitational energy whilst in suspension within the water, those with a higher specific gravity (density) settle faster than those with a lower count, resulting in a concentration of material with higher density at the bottom, on the jig bed. The particles are now concentrated according to density and can be extracted from the jig bed separately. In the mining of most heavy minerals, the denser material would be the desired mineral and the rest would be discarded as floats (or tailings).

There are some minerals, notably coal, that are lighter (lower in density) than the surrounding rock and in such instances the process of extraction would work in reverse, i.e. the coal would settle on top with the rock below (on the jig bed). There are several designs and methods of extraction from the jig bed.

WORKING PRINCIPLE FOR EFFECTIVE JIGGING

The Mineral Jig operates on the basic principle of hindered settling, and in order to effect a concentration it is necessary that there be a differential in the settling rate of the solids treated. As there is an accumulated differential in settling rates in the grinding mill discharge of a conventional closed grinding circuit, this is an ideal feed for the MINERAL JIG. In addition, this is an enriched product due to the settling and trapping of the higher specific gravity mineral particles in the classifier pool, and concentrate removal from the closed circuit is imperative.

In the upper sectional view the rotating water valve (synchronized with the eccentric shaft) has closed and the plunger is moving the diaphragm on its downward stroke which creates an upward pulsation to the JIG bed. During this period lighter gangue particles are carried further than the heavier mineral particles which may even settle against the upward movement in the pulp. At the completion of the upward pulsation the rotating water valve has passed through a 180° arc or half of a circle.

HOW DOES A JIG WORK

It is often hard for people to understand exactly how jigs work. For what it’s worth, I will put forth my understanding of the principles. Simply stated a jig is a device which has a couple of basic advantages over a sluice box. It is continuous and only needs occasional clean up (its biggest drawing card to hard rock mills).

It maintains, if properly balanced, an intermittent fluidized bed capable of retaining finer gold than a sluice box. If not properly balanced it can be more disastrous than a sluice box. A properly functioning jig is dependent upon a number of factors; feed flow density, feed flow rate, pulse rate, hutch water input, hutch concentrate delivery flow and of course the makeup of the gangue and specifically the particle shape and size of the gold.

However, the following are what we are attempting to do in a jig:

·   Create a cross flow on the top that:

a] is fast enough so that only the desired product and as little as possible worthless gangue accompanies it into the jig hutch and

b] is slow enough to allow the desired product to settle into the ragging without being rushed right over the top to tails for, once trapped in the ragging, capture of a particle is reasonably certain as all forward motion should now be defeated.

·   Put enough hutch water in to make sure that:

a] the negative pull of the diaphram or pulsing device does not create a sucking effect on the jig bed and

b] it’s not so much that all settling rate is defeated.

·   Draw off enough water with the hutch product to:

a] flow it to whatever point of delivery and

b] at same time balance this with the incoming hutch water to accomplish the aims  above.

If we analyze the foregoing statements, we must realize that if the incoming hutch water rate is too large, then the upward thrust during the positive stroke will be too severe and will push fine gold particles upwards and not allow them to settle out and if the incoming rate of feed and water is too large, then forward motion will be too fast and again not allow fine gold particles to settle out. Most of us know from experience that the settling rate of fine gold is quite slow so we can assume that the settling rate on the negative pulse of the jig mist overcame the forward movement turbulence of the gangue; for on the positive pulse, no fine sized gold particles will settle out. We can be quite certain that forward motion takes place on both negative and positive strokes albeit at different speeds depending on the portion of the pulse cycle it is in and depending on where the particle is in relation to the feed entrance and tails exit of the jig cell – for the flow rate speeds up the closer you get to the tails exit and in the same manner as a sluice box at same point the speed of the gangue destroys recovery of fine gold. One assumes that forward motion will be much reduced in the very last portion of the negative pulse but that settling rate is ranging from positive to negative in the positive pulse of the jig, and if there is the slightest bit too much hutch take up water the settling rate will go to the negative side sufficiently to stop all fine gold recovery and in the writer’s experience, some quite coarse gold recovery (up to 10 mesh in size).

Interstitial trickling is the redeeming feature of jigs for fine particle recovery. At the end of each settling cycle, coarse particles will bridge together first, and cone to rest. Finer particles will then trickle through the interstices of the larger one, and came to rest much later. Flaky particles are generally not well recovered by jigs. Firstly, their terminal velocity is much lower than that of a spherical particle of equal mass. Secondly, their shape restricts their motion during the trickling phase.

Jigs are often used in gold concentration. Their effectiveness, however, rapidly decreases below 300 µm (50 Mesh). This seriously limits their use as sole recovery unit. An interesting configuration is the placing of two identical sized jig cells, one behind the other. One of those jig cells absolutely has to be out of sync. If the first one is balanced, it will have sufficient hutch water added that the following combination will be in effect. In jig  solids will be correct but liquid content will be far too large, thus the forward speed of the total will be too fast for effective settling. Conversely, if volume and flow are correct for the second jig, then forward motion will be too slow in the first unit and either too much product will go to the hutch or blinding of the ragging and jig bed will take place. In essence, the same can be said of jigs as of sluice boxes. The large and dense particles will almost immediately drop out of flow and become part of the ragging. Like all 1 g systems the finer the gold and larger the aspect ratio the less likely you are to catch it. Jigs are not a fine gold recovery unit. Simply stated, unless you can find a, way to change the laws of the settling rate in relation to the laws of the flow rate you cannot improve the recovery beyond some fairly definite limits. The laws governing settling and laminar flow are quite absolute.