Anatomy of a Mill (Jigs)
After leaving the final stages of the sizing process, the copper ore that entered the mill as pieces of rock have been reduced to a coarse gravel and mixed with water to become a muddy concoction known as slime. Within that slime is a mixture of copper and rock of various sizes and shapes. Now it’s the mill’s job to remove as much of that copper from the mix as possible - in a complex multi-stop process known as separation. The first step in that process is carried out by a series of machines known as jigs.
Before we can take a look at how a jig works, its first important to understand why it works. Jigs take advantage of a particle dynamic known as the law of separation. This law states that a collection of various particles will naturally settle in water based on their relative densities and sizes. This means that the most dense and largest particles will settle first, with the least dense and smallest particles settling last. This creates a series of distinct layers, with the largest / most dense on the bottom and the smallest / least dense on top. For a mill, these layers usually settle out in relation to their copper content, with the most copper-rich particles on the bottom and the least copper-rich particles on top. A jig is a machine that helps hasten this process along.
In its most basic form a jig is simply a water-filled box divided into two compartments. One of these compartments houses a plunger, while the other is covered by a metal screen known as a sieve. During operation, copper bearing slime is poured onto the sieve, while in the connected compartment the plunger is repeatedly driven down and back up again. Because the two compartments are open to each other from below, this reciprocating motion is transferred through the water and up to the sieve.
At the sieve the copper-bearing slime undergoes two main hydraulic processes: pulsion and suction. During pulsion the water is forced up through the sieve and slime, which allows the natural separation process to occur. During suction, the water is drawn back down through the slime and sieve. This action causes the smaller particles of copper to pass through the sieve and settle on the bottom of the box. After several cycles of these processes, particles settle out of the slime and on top of the sieve in several distinct layers:
Headings (Copper Concentrate): The layer sitting right on top of the sieve are those particles with the highest copper concentration, and are known as headings.
Middlings (Ramblings): Middlings are a mixture of both copper and rock, and require further treatment to separate fully. A jig creates to types of middlings. The first is the layer of material that settles on top of the headings known as ramblings. The second is those pieces that manage to make their way through the sieve and collect at the bottom of the compartment. These are known as Hutchwork, and are emptied from the machine through a spigot.
Tailings: Those particles that settle on top of the ramblings contain very little copper and are known as tailings. Often these lighter particles are washed off the top of the sieve by a constant flow of water, and dumped into the waste launder for deliver to the lake.
Jigs are categorized by several means, but the types used most frequently in the Superior region were the Collom and Hodge types. Both of these were very similar, differing mainly in hutch design and the method by which the plunger was driven. A detailed look at the Collom design is shown below:
Unlike our simple model discussed previously, a Collom jig (and most jig designs really) is in fact two of our simple jigs in one. Here two plungers work independently against two separate sieves set on opposite sides of the machine. Water is delivered into the hutch by means of two inlet pipes. The copper-bearing slime is poured into the jig from the “head” side, with the tailings and middlings deposited out the “tail” side. Hutch-work which falls through the sieves and to the bottom of the hutch is expelled by means of a spigot at the base of each compartment.
In practice, each side of the Collom jig features sieves of different Mesh Sizes, which is a measurement of the amount of holes in each square inch of the sieve (A higher Mesh Size corresponds to a smaller hole size within the sieve). A mill will start with about four different sieve sizes right right off the bat (two Collom Jigs with two sieve compartments each). These four sizes correspond to the four sizes of copper-rock sent out from the hydraulic classifier.
In Superior area mills jigs are set up in two groups: roughing jigs and refining jigs. Copper rock leaving the stamps are sorted by size by the classifiers and sent to their corresponding sieve-sized roughing jigs. These jigs are often set up in pairs, similar to what is seen in the illustration above. Here the second sieve on the lower jig is of a larger Mesh Size (smaller holes) then the first. The slime makes its way way from the first jig down to the second, along the way settling out the heavier copper particles upon the sieves while the lighter particles are washed off. The lighter particles are tailings, and are simply allowed to wash into a waste launder and sent out to the lake. Every so often the jigs are stopped and workers scrape off the copper from the screens. At this time the middlings from both the sieves and hutch are sent on to the second set of jigs - known as the refining jigs.
Refining jigs are set up identically to the roughing jigs, only the sieves are of a larger mesh size (smaller holes). The same three types of materials are created, but the middlings created on these jigs from the screen and hutch are sent on to the wash floor for their final treatment. Together with the roughing jigs, the jigs sieves account for the majority of the copper extracted by the mill. With most of it’s larger particles extracted, the slimes original gravel like consistency is reduced to something resembling a fine sand by the time it leaves the refining jigs.
In the flow sheet above, 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.
Its that wash floor that we’ll take a look at next….