Milling, concentrating processes at Cliff Mine

Last week, we talked briefly about the process of mineral concentrating, beginning at the stamp mill. This morning, while we enjoy our coffee, we will back up a bit, and go into more detail, starting in the mine.

In drilling and blasting underground, the copper-bearing rock was blown out and onto the mine floor, in large chunks of various size, all of which were far too big to be fed directly into the feed box of the stamps. It would first need to be reduced after it was hoisted to the surface.

To make it small enough, the rock first went through a process called calcining. At the Cliff Mine, according to Harper’s New Monthly Magazine, for April, 1853, the rock was dumped into kiln. The kiln was a pit, roughly 14 feet square, sunk a few feet into the ground. The floor of the kiln was covered with a cast iron plates, upon which was stacked firewood to a depth of three feet. On that was placed the copper rock, to a weight of about 60 tons. The wood was then set afire and rock was literally roasted.

Once the fire had burned down and the rock was hot, men with sledgehammers pounded the rock, which because it was hot, broke apart more easily than if it was not heated. Harper’s did not mention it, but it was standard practice in calcining to pour cold water onto the heated rock, which would cause it to crack, making the task of size reduction much easier, as well as quicker.

Once broken into pieces weighing no more than two pounds, it was shoveled into a wooden cart on iron tracks, and trammed to the stamp mill.

The tram road angled downward from the kiln into the top of the stamp mill building, where it was then dumped into the pass. The pass was a large wooden box, 10 feet high and eight feet wide, and was attached to the back of the wooden frame that housed the two batteries of stamps, each containing five stamps. The length of the pass was the width of the stamp mill frame. At one end of the mill frame was a large, wooden pulley wheel that turned the camshaft. The cams, arranged in a particular, pre-set order, raised the stamps in a particular sequence in each battery, usually 1-3-5-4-2. When the belt that drove the camshaft was engaged, the stamps raised and fell in that pattern, crushing the rock fed from the pass into the mill.

The removable iron, 500-pound shoes on the bottom of the stamp stems, dropped onto the rock in what was called the mortar box. While the stamps were crushing the rock, a stream of water flowed into the mortar box. Across the front of the mortar boxes, was a wire grate, through which (when stamped into a course sand) the sand and copper particles, carried by the water flow, passed from the mortar boxes, into what Harper’s called “an angular space in front.”

“From this the heaviest portion tends to the small pool on the right; the balance passing off by the trough to the left, pass into and through the pits, which are two large receptacles just outside the of the stamp room.”

(While reading, it is important to keep in mind that copper is heavier, and has a greater density than the crushed rock. By using water in the process, the water, rock, and copper, continually flow through a series of processes that keep the the stamped material running always downgrade, at each step in the process, encouraging the heavier copper to sink beneath the water and the waste sand, and so separate the copper from the sand).

Even with the illustration above, that wording is confusing, unless you happen to already be a 19th century millman. However, in his 1955 book, Lake Superior Milling Practice, hydrometallurgist C. Harry Benedict simplified the wording, even though he used more technical terminology.

The “angular space in front,” Benedict explained, was a “small, two-way classifying section which diverted the heavier sands and copper into a pool, the overflow of which joined the lighter material of the classifier. The heavier copper left in the mortar hopper, and in the pool, were ready for barreling and shipment to the smelter. The lighter portions flowed into intermediate storage bins or pits. (What this classified the stamped material into what could be shipped to the smelter, and what needed further processing). The material needing further separation, or classifying, still requiring water, followed the practice of the Cornish tin mines, Benedict stated, which was a combination of “hand-operated jig, tie and buddle.” Do not let the terms confuse you.

The material coming to rest in the pit, he wrote, consisted of the heaviest and coarsest particles. It was subjected to a crude concentration process by shovel and broom, making a final tailing and a partial concentrate, or “middling,” which was reworked on the (copper washing) floor in a hand-operated jig.

The process here described was performed in a separate building called the wash house, in the floor of which were five long, narrow trenches. Into these trenches flowed the middlings, where the copper was further separated from the waste rock, or stamp sand.

“…a process of washing performed, technically termed jiggering.,” stated Harper’s. “The trench for this purpose is let some 12 or 18 inches into the floor and provided with a spring-board at the lower end, beneath the water, which greatly assists the boy who executes this process. The jigger is a kind of low tub, with a bottom of sheet brass finely perforated. A portion of the deposits from the heads of this trench…having been shoveled into the tub, the boy stands astride of the trench, and by sundry vertical and whirling motions in the water as well as out, gives it the proper degree of agitation.”

Here, we will stop for now, because we are out of space, and this is probably more than enough confusion for the mind for now. We will finish up the washing process next week.