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2023 | NOTES FROM THE UNDERGROUND | All About The Mill Edition

Greetings from Black Hawk and Central City, Colorado, our mining and milling hometowns, out there on the edge of the Front Range,

My last Notes included a video of the Ball Mill start up. Pretty soon I’ll be sending you videos of the Rake Classifier in action as well as the Coarse Ore Bin installation. Pretty exciting stuff, although one responder wondered how this all comes together and how the mill works. After all, rock from the mine goes in one end and gold comes out the other – so allow me to put it together. Consider the following Process Flowsheet that diagrams the rocks’ transformational journey.

You may need to enlarge your screen as you move through the Process Flowsheet. Each important step is numbered, and those numbers indicate the following:

1.Coarse Ore Bin - this is where the trucks from the mine unload the tons of ore bearing rock

2.Vibratory Screen - this allows the small, gravel sized material to bypass the crusher, increasing the crushing efficiency

3.Jaw Crusher – as the fearsome name implies, this machine crushes the rock into gravel size between its manganese steel jaws.

4.Transfer Belt – this is a conveyor that moves the crushed rock below the screen and crusher to …

5.The Bucket Elevator is a vertical conveyor with buckets attached to the belt which lift the gravel sized pieces, offloading them high up into…

6.The Fine Ore Bin which is the highest point in the mill and storage spot for the rock before the…

7.Belt Feeder, which carries the rock into the ….

8.Ball Mill. This is an important stop. Whereas the Jaw Crusher crushed the rock, the Ball Mill grinds it, turning it from gravel to wet sand, after which it hits the…

9.Trash Screen, which does exactly what its name implies, removing any foreign objects (wood, plastic, etc.) before moving onto the….

10.Mineral Jig, a gravity separation device which segregates the heavier metallic bits from the lighter quartz and rock sending a largely metallic concentrate to the finishing table. Any oversized pieces are returned to the Rake Classifier.

11.After being screened into three sizes (coarse, medium and fine), the jig concentrate is run across the Finishing Table. The Finishing Table is the spot from which the larger visible gold is removed, and a series of heavy concentrates are created of each metallic sulfide mineral. Gold separated at this stage can be melted or “fused” into doré bars to be sold to third party refineries.

12.The Rake Classifier is a sizing device which sends the oversized material back up and into the Ball Mill for further grinding. Properly sized sand is then delivered to…

13.The Conditioner Tank where the pulp (slurry of wet sand) is treated with the non-toxic flotation chemicals which allow the metals to sink or swim depending on their surface electro-chemistry.

14.The Reagent Feeders introduce the non-toxic chemicals at the proper dosage to achieve the flotation separation.

15-17. These are the series of flotation machines (think big blenders) in which nontoxic chemicals stabilize air bubbles that pick up and float the gold (and silver) which prefer to be dry (in air) to the top of the tanks. The non-metallic particles prefer to be wet and sink. The gold is held in the froth on the surface of the tanks where a series of paddles continually scrape the rising froth off the tanks and into the ……

18. Concentrate Thickener which removes water from the slurry before it is pumped to the Concentrate Filter which removes as much remaining water as possible from the gold flotation concentrate so that it can be placed in sacks for transportation to third party smelters.

19 – 22 contain the steps necessary to dry the tailings. Tailings are the sands that are left over after the valuable minerals and metals have been removed. Because these rocks contain minor residual sulfide minerals they can’t simply be dumped on open ground. Rain and snow would eventually cause the rocks to create sulphuric acid which could allow small amounts of naturally occurring elements in the sand (cadmium, arsenic, mercury, etc.) to be released into the environment. As a result, we concentrate, dry, and treat the sand with cement and foaming agents before returning it as cemented rocks back to the mine from which they came, thereby preserving the environment and stabilizing ground that would otherwise by prone to sink holes. So – that’s the story. When completed and in full operation, the Golden Gilpin Mill will be working 24/7 cranking out as much gold as the Bates Hunter has to offer. In the late 1800’s and early 1900s there were mines and mills throughout Central City and Black Hawk. Black Hawk was known as “The City of Mills” with thirty of them going strong. Over the decades, thirty became zero. The day we start milling will be a day history is made.





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