4KCy + 2Au + O + H2O = 2KAuCy2 + 2KHO
Cyaniding at Bodie
Michael H. Piatt
A new recovery process introduced in 1890 transformed the world of precious metals mining so successfully that variations of the method are still used today. Based on the well-established fact that gold and silver dissolve in a solution of potassium cyanide, the “cyanide process” brought new life and profits to tired old mining camps, first as a treatment for discarded mill tailings, then for processing low-grade ore from the mines.
Cyanide’s usefulness as a gold
solvent had been recorded through a century of advances and a proliferation of
patents since the 1790s. The first
commercial use of cyanide in the metal trades occurred in 1840, when English
gilding shops electroplated dissolved gold onto other metallic surfaces. An 1843 discovery in
Beginning in October 1887,
metallurgical chemist John MacArthur and two physicians, Robert and William
Forrest, secured a series of cyanide-related patents for the Cassel Gold
Extraction Company of
Mining men in
In an age and industry where wonderfully impractical ideas were pitched daily, the MacArthur-Forrest Process received a fair share of skepticism. One “practical miner” expressed doubt:
[in the pockets] of promoters. When they show me an honest miner who has got rich through this cyanide
process, I may take some stock in it. (Mining and ScientificPress 21 May 1892, 376)
Expecting to profit by collecting
royalties from authorized users, the Gold and Silver Extraction Company of
Throughout 1891 and early 1892,
assertions made for the process were challenged and defended in scientific and
trade journals across the country. Not
until summer 1892 did doubts subside in the face of persistent success
stories. The first cyanide plant in
MacArthur-Forrest Process. Although originally envisioned as an ore treating process, cyaniding found early worldwide acceptance for re-working old tailings. It promised to recover 70% to 80% of the gold and silver from the sandy waste material by using chemicals that cost only about $1.00 per ton of tailings. At Bodie, where tailings assayed around $5.00 per ton, there existed a good margin for profit.
Several district enterprises built
cyanide plants to process tailings from old mill sites. After completing its first plant in September
1894, the Standard Company conducted Bodie’s most extensive and longest lasting
tailings recovery operation. Standard
Tailings Plant No. 1 sat atop the Bulwer Tunnel’s dump, strategically situated
between the Standard mill’s tailings pond and a tailings pond below the
recently-dismantled Bulwer-Standard mill.
Horses and mules pulled plows and harrows to loosen dried tailings that
had accumulated during nearly 30 years of mining. Draft animals also dragged wheelless scrapers
called “fresnos,” similar to those employed in road and canal construction,
through the sandy material to scoop it up and carry it away. A long handle cantilevered rearward allowed
the driver to control the depth of cut and flip the
Leaching Vats. The process inventors originally contemplated large tanks with revolving arms to agitate a mixture of crushed ore and cyanide solution. This called for substantial motive power, and proved impractical in districts where fuel costs were high. Instead, Bodie’s early plants adopted a modified process that soaked granular tailings in wooden “leaching vats” and required only enough power to run several pumps. The vats were usually about 5 or 6 feet deep and 10 to 20 feet in diameter, each fitted with a false bottom made of porous textiles. A “filter bottom” consisted of layered wool packing, cocoa-matting, canvas, and burlap supported by wooden slats.(3)
At Tailings Plant No. 1, the horse-drawn equipment required about nine hours to fill one vat with 75 tons of dried tailings. A “strong” solution of cyanide and water flooded the charge from a pipe in the vat’s bottom. Although referred to as a strong solution, it contained less than 1% cyanide to minimize its action on base metals, such as lead, iron, and copper. Filling a vat with liquid consumed about four hours. The saturated tailings soaked for another four hours. Moisture from the tailings diluted the solution, so after soaking, the weakened solution was drained through the filter bottom (often with the aid of a suction pump), and displaced by strong solution until the ratio of cyanide was corrected. Periodic testing of the outflow determined when the liquid in the vat had reached the proper strength. Changing the solution and correcting its strength continued over the next 20 hours, during which each drawdown allowed air to infiltrate the porous charge with oxygen that hastened the action. When finally saturated with strong solution, the charge soaked another 20 hours. After the solution drained, a weak solution of about 1/2% cyanide filled the vat and soaked another 29 hours. That solution was drained and replaced with wash water to recover any remaining cyanide, requiring 13 more hours.(4)
All cyanide liquid drawn through the filter bottoms carried dissolved gold and silver, and was saved for further processing. Finally, a discharge gate opened on the vat’s side, and shoveling assisted by high-pressure water from a hose flushed the spent residue through sluices back into the tailings ponds. Emptying the vat took three hours. In total, each vat at the Standard plant required 102 hours, or about four days to treat one charge. The plant’s four 75-ton vats worked in rotation, one per day. Since one vat treated its charge every day, the plant was rated at 75 tons per day.(5)
Zinc Precipitation Boxes. The final steps in cyaniding removed solid gold and silver from the cyanide solution and melted them into bullion. Clear solution from the filter bottoms entered long wooden boxes divided into about 12 compartments. Partitions forced the liquid into each compartment from below, passing upward through perforated trays containing zinc shavings. Dropping as a gray-black powder, solid gold and silver particles precipitated when solution came in contact with the metallic zinc. The Standard’s plant employed five zinc precipitation boxes, three for the strong solution, two for weak solution. After giving up their precious metals, the liquids were stored for reuse in separate tanks, where additional cyanide corrected their strengths. The process consumed twenty pounds of zinc shavings daily, more than the amount of gold and silver recovered. The Standard mill’s lathe turned zinc discs to produce replacement shavings.
Bodie’s pioneering cyanide plants were built near old mill sites to take advantage of tailings that had accumulated during and after the district’s boom years: The Bodie Tunnel plant, near the Bodie Tunnel mill, presumably Bodie’s first--construction date unknown; Standard Tailings Plant No. 1, near the ruins of the Bulwer-Standard mill, 1894; Parr and Tyack’s plant, at the Syndicate mill, 1894; and the South-End cyanide plant, above Booker Flat, where the Noonday, Spaulding, and Silver Hill mills deposited their tailings, 1895. Cyanide plants along Bodie Creek treated tailings that had washed downstream: Standard Tailings Plant No. 2, located 3.3 miles below Bodie, 1896; University plant, 4 miles, 1896; Victor plant, 2.6 miles below town, across from the stone tollhouse, about 1897; Sunshine plant, 7.2 miles at the California-Nevada state line, 1898; the Baldwin plant, 14 miles down Bodie Creek at Fletcher, 1898; the Del Monte plant, 10.5 miles at the site of Aurora’s old Del Monte mill, 1899; and the Green plant, 18 miles from Bodie at the confluence of Bodie and Rough Creeks near Nine-Mile, 1900. The Standard Company’s third cyanide plant, built adjacent to the Standard mill in 1898, treated tailings directly from the mill as part of a combined ore milling and cyaniding operation.
Although remarkable for delivering profits, the cyanide process worked better on Bodie’s old tailings than it did on ore from the Standard Mine, which usually contained coarse gold, silver sulphurets, and slimes. Coarse gold particles required extended time to dissolve, slowing the process, so amalgamating plates in the Standard mill removed as much gold as possible. The mill’s mechanical concentrators then recovered the silver sulphurets, those vexing compounds that confounded most attempts to redeem their values, even with cyanide. Extended time in a separate pan and settler treated concentrates with a stronger than normal cyanide solution. Slimes, the very fine particles in tailings, complicated matters in other ways. Most apparent was their propensity to clog filter bottoms. They also tended to pack together in the vats, impeding percolation and sometimes forming impervious layers that brought filling and draining to a halt. These difficulties could be overcome to some degree by mixing slimes with granular tailings. The proportions had to be carefully controlled so that the blend remained porous. The precise ratio, however, accommodated only a small quantity of slimes, leaving many tons of the gold- and silver-bearing material in the ponds to torment experts who sought a method to profit from them.
Before Bodie’s pioneering cyanide plants were built, the Washoe Process had been the Standard mill’s primary ore treating method, until superseded in 1890 by amalgamating plates and mechanical concentrators. After cyaniding proved successful, General Manager Robert Gilman Brown abandoned the old pans and settlers in 1897 by linking the mill directly to the distant cyanide plant, where cyanide would recover gold and silver that had escaped amalgamation and concentration. Eliminating the expense of digging tailings from the ponds then transporting them to the cyanide plant, a bucket elevator raised the slurry above the mill, so that gravity carried it through a flume 1,800 feet to the cyanide plant. The flume was also expected to do away with winter shutdowns, when frozen ponds halted the horse-drawn excavating equipment. Cyanide would now become an integral part of treating mined ore, promising extraction rates between 80% and 90% of the assayed value--comparable to chlorination, smelting, or processes requiring roasting--but much cheaper. Cyanide was especially suited to districts like Bodie, where high transportation and fuel costs had forced miners to leave behind tons of poor ore that would have cost more to mine and mill than it was worth. Miners returned to the stopes to remove low-grade that now meant big profits.
Bodie’s first attempt to link milling with cyaniding, known as “direct treatment,” was a dismal failure. The flume, built in 1897, tying the mill to the cyanide plant frequently plugged with the gritty effluent and froze in winter. It was abandoned the following year, when the Standard Company built a cyanide plant adjoining its mill. Built for direct treatment, the plant may not have been operational before an 1898 fire destroyed nearby the mill.(7) The cyanide plant survived the fire, but was torn down in 1902 and its parts used to double the capacity of Tailings Plant No. 1.
In 1903 George Moore obtained an
American patent for a suction filtration system that dealt with the
conundrum. Suction pumps drew valuable
cyanide liquid through specially built filters that cleaned themselves. An enormous improvement over primitive filter
After months of research and testing, the Standard Company under Assistant Manager Theodore Hoover converted its enlarged cyanide plant in 1904 to the “Moore Slimes Process,” which promised to improve yields from slimes and tailings lying in the ponds. It would also increase extraction rates from crushed ore sent from the Standard Company’s new 20-stamp mill, built in 1899 to replace the mill destroyed by fire. An improved wooden flume, four inches wide by nine inches deep, carried pulp to the remodeled cyanide plant on a steeper downward grade than its failed predecessor. Four Frenier pumps, capable of withstanding the abrasive effects of finely crushed ore, pushed the pulp in stages 63 feet above the new mill’s floor through a vertical pipe, where it entered the flume. Trestlework behind the mill supported the flume as it angled downward toward the hill, then turned northward and descended to the cyanide plant 1,800 feet away.(8) Cyanide had again become the final stage of the milling process. To distinguish between tailings from the mill and tailings excavated from the ponds, the gritty product leaving the mill through the flume was referred to as “pulp.” “Tailings” and “slimes” described waste material excavated from the ponds.
Tube Mill. Settling tanks inside the rebuilt Standard cyanide plant mixed pulp (flumed from the mill) with tailings and slimes (excavated from the ponds) and removed the coarse sands, which entered one end of a “tube mill” that ground the granular particles to the consistency of slimes. Introduced to metallurgy in 1894, tube mills were the most recent grinding machines descending from Mexican arrastras and Washoe pans.
Essentially a large horizontal cylinder revolving on its
longitudinal axis, the tube mill tumbled very hard flint pebbles or iron balls
against the sand to be ground.
tube mill’s product and slimes were mixed with cyanide liquid, then conveyed to
one of nine wooden settling vats, each requiring about six hours to fill. After 28 hours of settling, clear cyanide
solution, now rich in gold and silver, was drawn off the surface and run
through zinc precipitation boxes for another eight hours to remove the precious
metals. Pumps transferred the murky
residue from the settling tanks into vats, where
The filter is lowered in the slimes and the vacuum pump started, the slimes being agitated to prevent settling.
The suction is continued until a coating of slimes is formed on all parts of the filtering surfaceto a depth of from 3/4 to 1 inch. During this time the pump is continuously discharging the clear gold solution. The crane then carries the basket to the ... wash water, the vacuum preventing the cakes from dropping off during the transfer. (Mining and Scientific Press 23 September 1905, 214)
Each gantry crane immersed its basket in the slurry for about 3-1/2 hours, while a vacuum drew cyanide solution through the hanging filter plates, accreting 18 tons of caked slimes. It was important that the cakes remained thin enough to allow cyanide solution to pass through. The gantry lifted the basket from the vat while vacuum held the cakes fast to the canvas. Encased with solids, the filters were conveyed to a wash water vat then submerged for another three hours. Vacuum drew fresh water through the cakes, recovering any cyanide solution still permeating the clinging colloids. After the filters were withdrawn from the wash water, 30 minutes of suction held the cakes in place and dried them, all the while recovering more liquid. Now cleansed of cyanide, the filters traveled to a discharge hopper, where the vacuum pump stopped and successive blasts of air from within dislodged the dried slimes, dropping them from the canvas. The entire sequence required eight hours for each basket. Throughout filtering, washing, and drying, all recovered liquid passed through zinc precipitation boxes that recovered the gold and silver.
The distant Standard mill received
several modifications to accommodate the
Inside the cyanide plant, workers had carefully marked cyanide pipes to prevent inadvertent poisoning of personnel. Despite all precautions, Theodore Hoover recalled nearly dying from accidentally drinking the deadly chemical. He had been hard at work on the remodeled plant, when he drank from a freshwater faucet.
In about five minutes I began to feel queer, but thought it was fatigue and loss of sleep. A minute or two later, I looked up, thinking that all the machinery had stopped, as everything was silent. The machinery was all running, but I had lost my hearing. Then my hearing came back, all right again with a bang. In a few minutes hearing was gone again, also sense of touch, and also, I suppose, taste and smell. By this time I was becoming excited, and my mind was working rapidly to fathom the cause. ... The cycles of lost hearing and touch were recurring regularly about every six minutes. (Hoover 1939, 136-137)
quick laboratory test of a sample from the spring-water tap revealed
cyanide. “After the first flush of
fright, however, I felt that, as I had lived the better part of half an hour
since drinking, the chances were that the poison would not get me that
Butters Slimes Process. Shortly after
Finely ground pulp and tailings from
the tube mill were mixed with slimes and agitated in a cyanide solution storage
tank, where ample time allowed the gold and silver to dissolve. The mixture was pumped from the tank into a
rectilinear vat containing nearly 100 hanging filter plates. After the rising slurry had engulfed the
plates, a vacuum pump drew the valuable liquid through the filters. Additional mixture had to be added as the
liquid was removed. After 3-1/2 hours,
caked colloids enveloped the filters.
Suction held the cakes in place while the tank drained and the pulp
returned to the storage tank.
1. Chlorination’s popularity was short-lived. Requiring large-capacity airtight tanks, expensive chemicals, and poisonous chlorine gas, the process recovered base metals more readily than gold and silver.
The new company found
that collecting royalties was not easy, a problem that led to its
downfall. Former MacArthur-Forrest
employees who understood the process presented themselves as “experts,” selling
their knowledge to mine owners seeking to avoid paying royalties by building
cyanide plants on the sly. An intriguing
silence from regional newspapers and the lack of other documentation suggests a
clandestine origin for Bodie’s first cyanide plant, presumably near the Bodie
Tunnel mill, whose construction date remains a mystery. Sixty years later, Ella Cain described her
father-in-law’s interest. “When the cyanide
process was discovered, J. S. Cain and A. J. McCone . . . hired an expert from
Some vats in
Lime added to the vats
helped check natural deterioration of the solution as it acted upon gold and
silver. Lime also reduced cyanide
weakening caused by organic plant matter dug up with tailings from creek beds
and the banks of tailings ponds. A kiln
5. A strange contrast occurred between stamp mills and cyanide plants. Solids contained the values in stamp mills, and water was discarded. Liquid, however, carried the gold and silver in cyanide plants, and the solids were discarded. Stamp mills were classified by the number of stamps, cyanide plants by their capacity in tons per day.
6. For detailed descriptions of Bodie’s four pioneering cyanide plants in 1898, Standard Tailings Plant No. 1, Standard Tailings Plant No. 2, South End plant, and the Victor plant, see: Francis L. Bosqui, Practical Notes on the Cyanide Process (New York, NY: Scientific Publishing Company, 1899), 36-192.
7. It is unclear if the Standard’s new mill was connected to the adjoining cyanide plant.
8. An iron pipe replaced the wooden box flume in 1909, and more substantial trestlework improved flow by straightening the pipe’s alignment.
For additional details
on the Moore Process at the Standard cyanide plant, see Robert Gilman Brown,
“Cyanide Practice with the Moore Filter.”
In Recent Cyanide Practice,
ed. T. A. Rickard (San Francisco, CA:
Mining and Scientific Press, 1907), 92-109. For a later account, see S. F. Shaw, “The
Standard Consolidated Cyanide Mill,” Engineering and Mining Journal (
Bosqui, Francis L.
Practical Notes on the Cyanide
Brown, Robert Gilman. “Cyanide Practice with the
Cain, Ella M.
The Story of Bodie.
“The Cyanide Process.” Mining
and Scientific Press (
“The Cyanide Process.” Mining
and Scientific Press (
Eakle, Arthur S., Emile Huguenin, and R. P.
McLaughlin. Mines and Mineral Resources of Alpine County,
“History of Cyanidation.” Mining
and Scientific Press (
Hoover, Theodore J. “Memoranda:
Being a Statement by an Engineer”
Shaw, S. F.
“The Standard Consolidated Cyanide Plant.” Engineering
and Mining Journal (