The progress made toward solving the problem of how best to treat the rather difficult ores of the Cripple Creek district is perhaps more striking to one re-visiting the camp after four years absence than to those who have watched its daily growth.
In any case the problem of ore-treatment at Cripple Creek possesses so much interest that no excuse is required for adding to its discussion.
The unoxidized ore at Cripple Creek consists essentially of tellurides of gold in a gangue of which the most significant feature is its high aluminum content. These give us the governing conditions ; the gold can not be recovered by amalgamation or cyanidation in the raw state, the ore can not profitably be concentrated because of the brittleness of the gold mineral, and can not be smelted except when mixed with a large quantity of easier-smelting ore.
Roasting is expensive and produces small lumps of metallic gold that are difficult to catch by amalgamation and impossible to dissolve in cyanide solution in any reasonable length of time.
Some years ago the problem had been brought to this stage of adjustment; the rich ores were sacked and sent to the smelters at Pueblo, Denver, and Salida (where some five years ago the smeltermen found that Cripple Creek ore could be used to advantage to replace the lime formerly used in the pot-roasting of lead ores to keep the charge from fusing in the pots).
Only the richer ores can be treated thus, as the treatment and freight charges become too severe a tax on the lower-grade material. That of medium grade was sent to mills at Colorado City where it was first roasted and then chlorinated.
The low-grade ore was left in the stopes or piled on the dumps.
Meanwhile Cripple Creek was fast approaching that stage in the development of every mining camp where the former medium-grade ore becomes high-grade and the former waste is regarded as low-grade ore, the change being due both to the exhaustion of the richer ore and the lowering of the working casts.
From time to time mills were started adjacent to the mines with the intention of treating lower-grade unoxidized material than could be shipped to Colorado City.
These were generally unsuccessful, for a variety of causes.
About four years ago the opinion had become general that the chlorination process, preceded by roasting, was unsatisfactory because of the high gold content of the tailing and the high cost of treatment.
Two improvements were made; the chlorination process was supplemented by concentration and by cyaniding the tailing; and a new process of roasting, re-grinding, running over blanket strakes to catch the free gold, followed by cyanidation of the ore.
In the face of the decreasing grade of the average ore, the items of $1 for freight to Colorado City and $0.70 (about) for roasting became painfully significant. It was clear that some method must be found of treating the ore near the mines, without roasting.
The problem of treating the unoxidized ore without roasting hinges on the fact that the gold telluride is not soluble in ordinary cyanide solution. It can be made to dissolve by using a solution that has great oxidizing power.
One of the first methods tried was bromo-cyanidation. There have been no published accounts of the detailed results of experiment on these ores, but the impression is general that bromo-cyanidation fell before the crushing blow of high working costs, coinciding in a general way with Western Australian experience in this regard.
It is not dead, however, as it is used in certain instances to supplement the main method of treatment, and I regard it as not improbable that with further experiment bromo-cyaniding may be found useful in many cases. The fact that the whole history of the cyanide process has been a constant struggle to perfect the mechanical equipment used must never be lost sight of in considering the merits of any proposed process.
A significant feature of progress in the past year has been the introduction of the Clancy process for treating unroasted ore. Its essential features are the use of cyanamide and iodide in the solution and the use of the electric current for producing a high degree of oxidation without excessive consumption of cyanide. The impression among the best informed is that the process obtains the desired extraction, though no actual working costs have been made public. A great deal of the discussion of this process has been footless and entirely beside the point.
Metallurgy is chemistry applied to business, and if by treating gold ore with a solution of old boots a good extraction at a low cost of operation can be obtained, it is good metallurgy; and if by treating with C. P. chemicals in apparatus of the latest refinement of design the extraction is poor and the cost high, it is poor metallurgy.
When the Clancy process demonstrates low costs of operation its position will be impregnable.
The present stage of development in treating the raw ore is to first crush in cyanide solution and classify the product into sand and slime, using either the Dorr or Akins classifier. The sand is concentrated on some approved type of table, the tailing rejected and the concentrate sent with the medium or high-grade ore to Colorado City or Pueblo.
The slime is treated by some process that will give the necessary oxidation. The most important mills are not yet ready to present their processes for criticism and comparison, but by the end of another year it is hoped that a more detailed discussion of progress can be made.
Meanwhile we have the brutal fact to face that the shipping ore from Cripple Creek is steadily decreasing in grade. This is partly offset by the fact that the constant pressure on the Colorado smelters to obtain silicious ores to smelt with the flood of basic ores produced in most of the Colorado camps makes them more willing to take Cripple Creek ore at a low treatment charge, even though it is aluminous rather than silicious.