|View single post by Keith Pashina|
|Posted: Mon Feb 13th, 2017 07:43 am||
|Gold Ore Processing in the Black Hawk Area
The whole point of the Gilpin Tram was to economically haul gold-bearing ore down to the ore-processing mills located in Black Hawk. The railroad drastically reduced the cost of hauling ores down by horse-drawn wagons. But more importantly, the railroad allowed lower grade ores that had previously not worth enough to justify haulage by wagons to be hauled by rail.
Gold ore processing methods changed over time, in part due to changes in the ore chemistry, and in part due to advances in technology. For the time period I am interested in, when the Gilpin Tram operated, the ore handling methods used were similar at the various mills near Black Hawk.
During the first mining booms in Gilpin County, the miners were at first encountering free gold near the surface, and when they started pit or shaft mining, were encountering surface ores, which were for the most part oxidized and easily milled using stamp mills and amalgamating tables. The “Mining and Scientific Press” on December 4, 1921, reported that the first homemade wood stamp mill was built in 1859, and a later that year a “modern” imported 3-stamp plant was erected. This technology took off fast, and there were 60 stamps in operation by 1860. But, as the mines deepened, the ore chemistry changed to containing more and more sulphides compunds. This was a disaster at the time. While the milling practices had been able to recover as much as 75% of the gold formerly, the same equipment did not work well on the deeper sulphide-containing ores, and the recovered gold contents at the mills dropped precipitously, to 30% or less!
The above section shows what could be considered a typical stamp mill for precious metals - at least as far as we modelers are concerned. However, the typical mill, one that cascades down the hillside, was not that common down in the Black Hawk area
The solution to the previous milling problems was studied by several ingenious inventors and tinkerers who eventually developed a modified process that was customized for the Gilpin County gold-bearing ores. These processes are more technical than what I, a non-mining type person, can fully understand. And, since my interest is a hobby, not professional, I cannot nor do I need to know all of the intricacies of how the ore was processed. However, what I am interested in knowing is:
· Basic knowledge of how ore was processed, so I can understand how the mills were laid out and constructed, and,
· What machinery was inside the mills, so I can build models of it
So, with some simple parameters set, let’s look at the how and why of the gold ore processing milling practices near Black Hawk.
A disclaimer: I will greatly generalize, and simplify, this discussion. I know several regular FreeRails readers have mining backgrounds, so if any of you would be kind enough to offer more detail and information, please do so!
In previous posts, we looked at the Hidden Treasure Mill. This mill had many interesting features that would make a fascinating model. We will look at this mill even closer, because several mining and technical publications in the early 1900s also provided a lot of information about how this mill operated.
This is a somewhat modified excerpt from an 1895 Sanborn Fire Insurance Map. This map was apparently drawn when the mill had grown to 75 stamp heads. This was a drastic enlargement from its earlier history, and in later years, was modified further