Anatomy of a Mill (Final Sizing)
The process of ore production neither begins or ends at the stamp mill, but instead stretches from the dark drifts of the underground right up to the smelter docks. This process consists of three main stages: sizing and sorting, separation, and refining. The first stage begins at the mine itself, where the miners work to break down the copper bearing rock first into pieces large enough to remove from the underground. From there these pieces are further sized at the rockhouse, where crushers and drop hammers continue to break the rocks down. Along with sizing, the miners and rockhouse also serve to sort the ore - sending copper poor rock out to rock piles while sending copper bearing rock out to the mill. Once the ore arrives at the mill, steam stamps work to reduce its size even further.
Now we take a closer look at the tail end of that sizing and sorting process, which occurs shortly after the ore leaves the stamp’s mortar boxes. The path this ore then follows is laid out in the diagram above. Keep in mind that every Superior Stamp Mill was slightly different in its layout and approach. The diagram above has been simplified and generalized for the sake of clarity.
After having been crushed down to a small enough size to fit through the mortar screens, ore makes its way to a machine known as a Trommel. The trommel insures that only ore smaller then a specific size is allowed through, sending anything larger back for re-processing (at either a set of rollers or back at the stamp itself). After successfully passing through the trommel, the ore then moves on to a classifier. The classifier works to sort the ore into several different classes - based on its copper content. Here copper-rich ore is set aside for transport to the smelter while copper-barren rock is dumped into the lake as tailings (stamp sand). Everything in between is sent on to the next step in the process - separation.
Let’s take a closer look at each of these steps, starting at the stamp itself…
Mortar Discharge
For a time there was only one path for ore to take out of a stamp’s mortar box - the screens. While most rock would end up passing through the screens just fine, some of the larger pieces of copper would not. Unlike the surrounding rock which is fractured by the action of the stamp head; larger pieces of copper are merely mushed into flat pieces within the mortar. These flat pieces of copper would clog up the screens and require periodic cleaning of the mortar box. Since this required a stamp to be shut down completely (and thus effecting production), mine companies looked for methods of removing that copper without having to shut the machine down. Thus the mortar discharge was born.
A mortar discharge is simply a outlet tube, attached to the mortar box at a point several inches up from crushing surface. Water from within the mortar box (which would be filled up almost completely during operation) would drain out this pipe. A second current of water - counter-acting the drain flow - would be sent up the pipe and into the mortar. It is this current of water that determined what would be allowed through the discharge.
The action of the stamp within the mortar box created a great deal of turbulence and wave action within it. This action would push rock pieces up against the pipe while the stamp was operating. While heavy pieces of copper could fight through the upward current in the pipe and make their way down the discharge, lighter pieces could not and where pushed back into the mortar. The copper that made its way down the pipe were delivered to a mineral bin, to await transport to the smelter. The mortar discharge proved highly successful for the mine companies - contributing over 10% of all the copper harvested at a mill.
Trommel
For the remaining rock and copper leaving the stamp via its screens, the next step was a sorting machine known as a Trommel. In the Lake Superior Region, these machines took the form of a revolving cylinder covered in a perforated steel plate. Ore and water would enter the machine on one end, making its way down the length of the cylinder due to a slight downward slope. The rotating motion would force the ore to spread out and ride up the sides of the cylinder. As the ores made its way down to the opposite end of the cylinder, any ore small enough to pass through the holes would be collected. Anything larger would make its way to the end of the cylinder, where it would be sent back to re-processing. This material was known as oversize, and the material passing through the screens was referred to as undersize. It would be the undersize that made its way on to the next step.
The trommel was driven by a large pulley wheel at its end, which were turned by the mill’s overhead belt and pulley system. A trommel’s screens wore out often, more so on the head side (nearer the stamp) then the tail side. The most wear would occur at the spot where the ore would fall into the machine, which resulted in the addition of a stronger non-perforated plate there (known as a dead plate) to absorb the impact. Even so, the screens of a trommel would still need to be replaced on a constant basis.
Classifiers
Before moving on to the separation stage, the ore leaving the trommels must first be sorted based on size. This is done in a machine known as a classifier, or more specifically a hydraulic classifier. This was essentially a deep trough roughly 15 feet in length and a foot deep. Ore would enter one end of the classifier suspended in a moderately paced flow of water. This water/ore solution would make its way across the trough and out the other side. Within the trough were 2 or more (4 in our example above) partitions, divided by only a short wall. Within each of these partitions would rise a column of water (a hydraulic flow), fed by a water pipe in the base of the trough.
These columns of water are the driving force for the sorting that takes place within the classifier. As the flow of water horizontally through the trough progresses, ore particles naturally began to fall out of that flow and settle towards the bottom of the trough. The upward columns of water help to keep aloft those ore particles, with only the heaviest and largest pieces being able to fight the current and settle out. For each subsequent partition within the trough, that upward column of water is reduced in power. Because of this, smaller and smaller particles are allowed to settle out in each subsequent partition. This created four partitions with four separate sizes of ore settled out along the bottom, from heaviest (and largest) to lightest. This ore is then drained from the bottom of each partition through spigots, which send the product to separate destinations based on their position within the classifier.
A classifier is calibrated to insure the heaviest particles settling in the first partition are known to be mostly copper. These are set aside in a mineral bin for transport to the smelter. The classifier is also calibrated to insure the lightest particles settling in the last partition (or running out of the trough without settling at all) are known to be rock, and are dumped into a launder for transport to the lake as waste. For everything in between, each separate classification is sent to a set of roughing jigs for that specific weight and size of ore.
We take a look at the jigs next….