The discovery of the Comstock Lode in Nevada in 1859 had attracted great numbers of California miners away. Actual progress in sinking on the veins was slow. The Hayward, now the Old Eureka mine, being worked by Central Eureka Mining Company at Sutter Creek, had reached an inclined depth of 1230 feet, and was the deepest mine in the state in 1867, having been in continuous operation since 1852. The Eureka mine, now part of the Idaho-Maryland at Grass Valley, was only 100 feet deep in 1863. The North Star mine shaft at Grass Valley, probably the deepest in that district at that time, had reached an inclined depth of 750 feet in 1867, and the vein had been explored underground for a length of 1000 feet. At last, quartz mining was finally becoming firmly established.
Innovations and
Improvements:
The early-day shafts were sunk as inclines following the veins on the
dip. Steam engines were used mostly for power. Hand drills and black powder
were used entirely until 1868, when the first air drills and dynamite were
introduced, although the widespread use of air drills for sloping ore did
not come for 30 years. These last two innovations did much to increase the
speed of underground development, resulting in lower gold content per ton,
and called for more mill capacity. The gravity stamp was gradually increased
in weight from 250 pounds to 1250 or 1500 pounds each. Self-feeders,
actuated by one stamp in each battery of five, were provided. Before 1870,
the foundries of San Francisco, particularly the Union Iron Works, had
become recognized as makers of
stamp mills and it was in this state that these mills were
brought to the high standards that made them the principal means for
reducing
gold ore for 50 years. Square wooden stems were supplanted in
the middle sixties by cylindrical stamp stems of iron fitted with tappets
which, when engaged with cams on the horizontal power shaft, lifted and
rotated the stamp. Outside
amalgamation was provided by long plates of copper, which were first
silver plated, then coated with a thin layer of liquid mercury. Rock
breakers, such as the Blake crusher, were introduced in California in 1861
to prepare ore for the stamps. The self-feeder, the rock breaker, heavier
stamps and increased running speed gave the stamps greatly increased
capacity.
Quartz mining differs much from placer mining. For the former, more capital, more experience, more complicated machinery and richer material are required than for the latter. The placer miner throws the dirt into the water, which then does the work; whereas the pulverizing of rock is a nice operation, requiring constant attention. Quartz requires a mill and water power; placer dirt is washed in a simple sluice. Dirt containing ten cents in the cubic yard may pay the hydraulic miner, but the quartz miner must have a hundred times as much in a cubic yard of vein stone, or he cannot work. The placer gold, when freed from the baser material surrounding it, is much of it in coarse particles, which are easily caught by their specific gravity; the quartz gold must be reduced to a fine powder before it be set free from its gangue, and with the fineness of the particles increases the difficulty of catching them. The rich quartz-veins of California extend from Kern River to the Siskiyou, are found on hills, in canons and in valleys.
Treatment of the
Concentrate
The concentration of the
sulfides, which makes up 1 percent to 2 percent of most of
the California gold ores and nearly always carries enough
gold to be worth saving,
was first made in sluices with riffles, similar to those of the placer
miner. Typical sulfides present were pyrite,
galena,
chalcopyrite and
arsenopyrite. In the sixties, Cornish buddies and other
European devices were used, and new types of concentrators were invented. In
July 1867, George Johnston and E. G. Smith patented an endless belt vanner
of rubber, set on a slight incline between rollers 8 or 12 feet apart. It
was driven slowly toward the upper roller and also had a short, rapid motion
sideways. It proved so effective in saving concentrate that it and the
similar Frue vanner, perfected in 1878, became standard equipment where
stamp mills were used. The chlorination process, previously known in Europe
as the Plattner process, for recovering gold from the sulfide concentrate,
was brought to California and improved to the point where it was widely used
until cyanidation was introduced in 1896. It will be seen from these side
lights that California quartz mining and milling practice drew freely from
European methods, which were usually improved and modernized here. The
mining engineers and metallurgists of this state became so well known for
their work that their advice and services were sought when new districts
were opened in foreign countries.
Development of Water
Power
For a long time after quartz mining started, steam engines using wood
fuel obtained locally were the principal source of power for mines and
mills. The development of large water systems for hydraulic mining, and the
increasing cost and scarcity of wood for fuel, led to the use of water
power, under high pressure, to operate tangential or impulse wheels. By the
middle eighties, nearly all of the principal quartz mines were using Knight,
Pelton or Donnelly wheels of the impulse type to supply their power needs.
Of the three makes, the Pelton wheel has been most successful, and the
Pelton Water Wheel Company is still a going concern in San Francisco, their
product being used in later years for generating hydroelectric power. This
Pelton wheel is highly efficient, and made possible the introduction of
electric power in the mining regions of the West before public utilities had
entered the field. The first recorded use of electric power for the
operation of mining or quartz
stamp milling machinery in California was at the Dalmatia
mine in El Dorado County in February 1890. Water under 112 feet pressure was
delivered to a Pelton wheel 7 feet in diameter, which operated an electric
generator of 126 horsepower, and current was transmitted over a line about
one mile long to the plant. Pacific Gas and Electric Company has
acknowledged its debt to the California miners and the tangential water
wheel. It is not too much to say that the company owed its origin to the
hydraulic miners and the Pelton wheel. Lode mining of gold became important
in the 1860's, and between 1884 and 1918
gold quartz veins were the major source of California's gold
production.
Cyanide Treatment To Recover Gold
From the time of the introduction of the process in about
1895, treatment of quartz ores with cyanide revolutionized the industry,
greatly increasing the recovery of gold from the ores. Because of its
economy, the increase in output from cyanide plants spread rapidly through
the industry. Previous to the invention of this process, an occasional heavy
thundershower, which carried a few thousand tons of tailings down the
creeks, was looked upon as a perfect godsend, but afterward, every ton of
tailings was treated as a valuable asset from which gold and silver could be
recovered with cyanide. The process works as follows – the ores are treated
by the normal means, and the tailings set to a separate part of the plant
for cyanide treatment to recovery the
gold missed in the first
process. Vats are filled within an inch or two of the top with the tailings.
Sufficient solution of cyanide of potassium to thoroughly saturate and cover
them is run onto them. This generally means about one third of the dry
weight of the ore. The first solution thus applied (the "strong" solution)
may be of 0.3 per cent, strength. It should always be borne in mind that the
weaker the solution employed the more the particularly selective action of
the chemical comes into play. In other words, as long as we use a solution
just strong enough to dissolve the gold in a reasonable time any additional
strength is a waste, in that compounds of iron and other substances, or even
the atmosphere, will destroy a proportion of the chemical in a strong
solution, when they would not act on weak solutions.
After standing say for 12 hours, during which time small quantities of the solution may be drawn off occasionally to effect artificial diffusion, all this solution is drained through the zinc boxes until the tailings are dry. A weaker solution say 0.15 per cent and about one half in quantity of the first solution is then run on, and may be started draining within an hour or two. A further weak wash, or, if the quantity of stock solution will allow, a water wash, is then applied and drained off. After the tank is thoroughly drained dry it is ready to discharge The solutions that have drained through the zinc boxes into the storage vats, or have been pumped to the upper reservoirs, are ready for making up to strength for further use on next charge, practically all the gold having been extracted from them in their passage through the zinc shavings. This treatment applies to a class of ore which may be considered as rather an exceptionally simple one, but was fairly common type of gold ore in California. An average extraction of 90 per cent, of the original gold contained is nothing uncommon in dealing with this class of ore.
In order to overcome the cyanide-destroying qualities of the acid or iron salts present, sometimes operators were obliged to neutralize, by means of an alkali or alkaline earth caustic soda or lime with or without a preliminary water washing to remove such soluble "cyanicides" (cyanide destroying chemicals) as may be present. While modern Merril-Crowe cyanide plants use powdered zinc, old time operators used zinc shavings as follows: Having obtained the gold in solution, the precipitation in the zinc boxes is, as a rule, a simple matter. The only points requiring attention are to keep a sufficient stock of zinc shavings in the compartments, and so to regulate the flow of solutions as not to incur danger of fine gold precipitate being carried away. The zinc shavings are prepared by turning down zinc discs on a lathe. The discs for this purpose maybe cut out of No. 15 gauge metal, and may measure from 6 to 12 inches in diameter, a hole being punched in the centre for the mandrill. It is usual to put bundles of 20 such discs on a mandrill. The speed of the lathe may be anywhere from 150 to 350 revolutions per minute, and the shavings are turned off by hand with an ordinary carpenter's mortise chisel.
Precipitation of the gold varies somewhat with different classes of ore treated. The completeness of the precipitation appears to depend in a measure on a slight excess of cyanide of potassium being present in .the solutions. Roughly speaking, it may be said that if solutions leaving the zinc boxes assay more than 2 dwts. per ton, the precipitation is not as it should be. This may be owing to the paucity of zinc in the boxes (which should be instantly rectified), to too great a speed in the flow of the solution, or, in very exceptional cases, to insufficient cyanide in the solutions. The zinc shavings in the boxes may require replenishing every day to replace amount consumed, or may run a week at a time without requiring dressing. Having, by passage of the solutions through the zinc shavings, re-converted the gold into the metallic form, as a sludgy-looking precipitate commonly known as "slimes," the next step in the process is to get this precipitate into marketable shape. This is done by separating from the zinc, drying, roasting, and smelting. The "cleaning-up," which takes place once or twice a month, is conducted as follows: A sufficient amount of clean water is run through the boxes to remove the cyanide solution, which might otherwise be injurious to the workmen. The zinc shavings are taken out, being twisted and rubbed in the water to remove, as much as possible, all gold adhering to them. In some cases there is quite a thick gold plating on the zinc, which cannot well be removed by scrubbing, but as all the shavings are returned to the boxes anyway, and this plating of one month will go to form the precipitate of the next, it may be ignored. Having removed nearly all the coarse zinc, the precipitate contained in the water is allowed to settle the addition of a little alum will considerably expedite the settling. The bulk of the clear water is then siphoned or pumped off, and the precipitate, most of which is under the trays, together with the remaining small quantity of water, is drawn off through the plug-holes into a calico or linen filter, or into a filter press. After drying sufficiently to handle with a scoop the precipitate may be further dried in iron pots, and is then ready for roasting and smelting.
Continue on to:
Quartz Lode Mining Systems, Part I
Return To:
Hard Rock Quartz Mining and Milling
