Historic Structures

Development of Gold Milling Technology Bald Mountain Gold Mill, Lead South Dakota

In tracing the line of development from early gold milling techniques used in the Black Hills through the technology applied at the Bald Mountain mill, several trends emerge. As the most accessible and profitable ores were worked out, fresh deposits were exploited. Each variety of ore required solutions to new problems and spawned successive generations of technological change. These generations of technology provided continuity between the phases of the Bald Mountain mill. During the short history of gold milling in the Black Hills, the rate of this technological change has been remarkably swift, perhaps due to the pace of operation and intensity of exploitation that characterizes gold fields. In addition, the majority of techniques and machines have arrived, at least partly developed, from other gold fields and then been adapted to local conditions.

The earliest gold deposits recovered were of the "placer" type - gold eroded from ores close to the surface and found in river bed gravel. The heavier metallic content of this gravel was recovered by panning. Several preparation methods were used, often essentially consisting of using shallow troughs with transverse ridges in the bottom to catch the heavier gold. This basic principle proved to be remarkably enduring. Inclined tables covered in coarse corduroy were still used in conjunction with more sophisticated techniques in major mines of the 1920s. Such devices could use mercury amalgamation to catch the finest gold dust. In this process mercury dissolves gold and silver to retain them in a solid, amalgamated mass. Mercury placed in the bottom of the troughs would retain exposed gold which flowed or landed on it.

As "free milling" ores (those requiring relatively simple milling techniques to release the gold and silver) gained prominence over placer as the main type of deposits being worked, mercury amalgamation was also used in mills. To enable the mercury to amalgamate ore was crushed to expose the metal content and then passed over copper plates coated with mercury. A later development was the amalgamation pan, where ore, in slurry form, flowed through large mercury coated pans while being mixed with the mercury by rotating blades. The resulting amalgam collected at the bottom of the pans to be removed later. The bullion was then extracted by distillation or filtration.

To crush ore many early mills used an arrastra, a large circular stone running around inside a cobbled trough. This method was eventually superseded by stamps. The stamp consisted of a hammer head fitted to a vertical shaft that fell on ore placed in a mortar beneath. Stamps were frequently arranged in large "batteries." Arrastras and stamps could both be used to combine the crushing and amalgamation processes by working the ore wet with mercury. A typical stamp mill of the 1880s shows several similarities with the earliest phases of the Bald Mountain mill. Mine cars deposited crude (unprocessed) ore into large bins. From here it was fed to primary crushers designed for the heavy work of breaking the large pieces of ore. This crushing was done with the ore still dry. From the crushers the ore moved to the stamp batteries, sometimes with mercury added to the mortars, though possibly just with water. After stamping the ore passed over the amalgamation plates where the amalgam was collected. The bullion (gold and silver mixed) was recovered by heating the amalgam beyond the boiling point of mercury in sealed retorts. The vaporized mercury was drawn off and condensed in a water bath for reuse while the bullion was refined.

The mercury amalgamation method, however, was unable to recover bullion from the refractory ores that contained gold and silver in complex physical and chemical bonds, requiring more sophisticated techniques to release them. As the free milling ores found closer to the surface began to be exhausted, new technology was required to process refractory ores.

One solution to the problem of milling refractory ores was the chlorination process. The chlorine process was invented in 1848 by C. F. Plattner in Germany. Finely crushed ore was mixed with chlorine and sulfuric acid diluted in water and placed in chlorination barrels. Chlorine gas was produced and gold was dissolved. The solution was then drawn from the bottom of the barrels and pumped to precipitation tanks. In these tanks a ferrous sulphate was added to precipitate the gold from the solution. Alternately, other metallic sulphides, hydrogen sulphide gas or charcoal could be used. The resulting gold precipitate was separated from the chlorine solution and placed in filters, where it was pressed between the leaves of filter bags until fluids passed through and solids were deposited on the bags. The filter presses were periodically cleaned and the precipitate dried and roasted with fluxes to remove slag and release the bullion.

In order for chlorination to work effectively, ores had to be finely crushed by a secondary crushing facility, duplicating the role of stamps at an amalgamation mill. Although stamps themselves were still used, new types of rotary mills were increasingly employed for finer grinding. In the case of refractory ores like those found in the Black Hills, heating in rotary roasters was often required to weaken the physical bonds of the metal and rock before the application of chlorine.

Although the chlorination process was successfully used on the refractory ores of the Black Hills, one major disadvantage of the process was that it could not recover the silver contained in these ores. Moreover, the chlorination process was always expensive and the advent of cyanidation quickly caused its demise. Cyanidation proved to be a more enduring method of processing Bald Mountain refractory ores. It is important to note, however, that in the chlorination mill the basic sub-divisions of process that characterize the cyanide mill can be seen: coarse crushing, milling (fine grinding), chemical action in tanks and precipitation/ filtering.

The cyanidation process was developed in 1887 at Glasgow University, Scotland, although the principle had been outlined as early as 1846. Potassium cyanide was originally used but sodium cyanide, a cheaper alternative, became standard. In dilute form cyanide will dissolve gold and silver from their ores and carry them in a solution from which they can be precipitated. The original patent also included the use of zinc as an agent in precipitation as zinc attracts the sodium in the solution, allowing the metals to be released, collected and smelted for bullion. Aluminum dust, charcoal and electrolytic action were used as alternative precipitation agents. The success of cyanide's dissolving action can be influenced by the acidity of the pulp (crushed ore and cyanide solution). Several additives could be used to control this factor - crushed lime was applied at Bald Mountain.

This process was first practiced commercially in New Zealand (1889) and reached the United States with the Consolidated Mercury plant in Utah (1891). Cyanidation was an immediate success in many areas, though in the Black Hills adaptation to localized conditions at first reduced the effectiveness of the process. The Homestake mine, in Lead, South Dakota, was the first to bring the technique to the Black Hills in 1900. Cyanide was initially used as a leaching agent. Dilute cyanide was added to large tanks of crushed ore, passing through the solid material and dissolving the metallic content into a solution which could be collected, filtered and finally precipitated to recover the bullion. In this form cyanidation had much in common with the chlorine process. To be useful for leaching, ore had to be milled to sand size (approximately smaller than 0.00714") and more effective ball, rod and roller mills were increasingly used in place of stamps for the fine crushing process. Cyanide was added at the milling stage, in a similar way to wet stamping with mercury. Each sand tank received several washes of cyanide. The solution was drained from each wash, filtered to remove any solids and the bullion removed by precipitation.

From the first application of the process it had been clear that improved dissolution could be gained by treating ore crushed to slime consistency, (smaller than 0.0025"), and that some ores required this level of crushing to enable cyanide to dissolve their metals. To make this method economical proved to be a difficult process and many mills adopted sliming technology slowly, not having the finances to risk an all-sliming operation with low-value ores.

In mills both partially and fully practicing slime treatment, it was necessary to develop various types of classifiers to separate slime and sand. J.V.N. Dorr's rake classifier was developed in the Black Hills in 1904. It separated the two constituents by their relative rates of settling. When mills turned to all-sliming production, the sand discharge end of their classifiers could be connected in closed circuit with the fine grinding equipment (usually a rod or ball mill) in order to contain the sand until it had been ground to slime. The use of ores processed to slime grade was increasingly common with the use of improved milling technology, often in closed classifying circuits to trap the ore and ensure fine grinding, or with a secondary stage of coarse crushing added. By 1936, sand leaching was "seldom incorporated in new plants."

For the cyanide's dissolving action to work effectively, oxygen is needed within the mixture and the solids have to be kept suspended in the dilute cyanide. By agitating the mixture both needs can be met. Early agitators used mechanical action with compressed air applied later. In 1907, Dorr combined the two in his design. Another problem was to separate the gold and silver bearing liquid from the solids. Dorr's thickener of 1905 enabled solids to settle and be removed by a drain while liquid was drawn off. By arranging thickeners in sequence, solids could be moved to each successive tank, releasing their bullion content to solution as they proceeded.

The counter-current method was a variation of this system that was in common use by the 1920s. The bulk of the solution in the thickeners did not flow in the same direction as the thickened solids, following them from tank to tank, but moved in the opposite direction. This arrangement meant that a solution with a low concentration of metal, or completely free of it (barren) would become gradually enriched as it moved through the successive thickeners. The solid would be in contact with the liquid when the former was at its strongest bullion concentration and the latter its weakest (hence, most chemically active). Before bullion was precipitated from the pregnant solution, filtration was used to remove suspended solids. Early filters using a compressing action were superseded by those using a vacuum to remove the gold and silver solution from the solids. Rotating filters were followed by the highly successful vacuum leaf filter (known as the clarifier) invented by Moore and Butters in 1903.

Despite the use of occasional alternatives zinc remained the most effective agent of precipitation. The zinc box, an early method of precipitating bullion by bringing solution into contact with zinc shavings was, Dorr claimed, adopted partly to avoid the patent specification of zinc dust as the precipitant. A more effective system of precipitation was to feed zinc dust directly into the solution.

It was discovered that precipitation would be more effective if the oxygen that had been introduced to aid the dissolution of metals during agitation was removed. A process developed by T. B. Crowe around 1907 passed the solution through a vacuum chamber to remove the majority of the air. Zinc dust was then added to the de-oxygenized liquid. In the commonly used Merrill process, dust was added to only a small part of the solution which was then returned to the main flow of liquid. The precipitate was collected in either filter presses or in filter bags, suspended in the solution. It was cleaned from these and taken to be dried and melted to release the bullion.

Throughout the development of gold milling techniques, certain similarities of function persist and are found in each successive generation of technology. In terms of coarse crushing of the ore, a continuity can clearly be seen in the amalgamation, chlorination and cyanidation techniques. In all three coarse crushing exposes the metallic content of the ore to prepare it for the chemical action, but in the amalgamation and cyanidation processes this chemical action also takes place during the wet milling stage.

There is also a link between plate amalgamation, barrel chlorination and cyanide sand tank leaching. In all three bullion and a chemical agent are brought together and bond without mechanical action to yield a combination of the two. Although the precipitation stage only dates from chlorination technology, it was continued and developed in cyanidation. The final production of bullion by forms of smelting is common to all types of mill- based gold operation.

Bearing in mind this smooth flow of progress, it is important to note that cyanidation marks a stage of increasing complexity in gold milling. Although the basic production stages were maintained, an important degree of sub-division took place during the process' first few decades. Coarse crushing became divided into primary and secondary; the non-milling chemical stage was broken into thickening, agitation and secondary thickening, and the precipitation stage saw the addition of the Crowe vacuum process. However, the cyanidation technique cannot be seen as separate from its predecessors. Links between techniques were not only historical: in many cases cyanide mills processed ores with a significant amount of coarse gold content and incorporated a mercury amalgamation facility into the production process. At the Bald Mountain mill a similar overlap in generations of technology can be seen between the sand leaching and slime decantation systems.

This underlying continuity of process means that the growth of the Bald Mountain mill occurred by altering or extending existing production areas rather than adding completely new ones. Consequently the mill has undergone mainly lateral expansion of the structure to accommodate increased output, rather than linear expansion along the flow of production. The machinery used has also followed the three primary phases of cyanide technology development: sand leaching, mixed practice and all-sliming. The latter was refined to a counter-current decantation system.