Description of the Process of Working Comstock Silver Ore.
In speaking of the works at present in use for the reduction of silver
ore, it will only be necessary to describe the process in use in one
mill, as all work after the same system. Being the most recently
erected, and quite perfect in all its arrangements, the new mill of the
Nevada Mill and Mining Company, commonly called the Chollar Mill (as it
stands near the Chollar old shaft), shall furnish the illustration
necessary to an understanding of the method of working Comstock ores now
generally in use. The mill covers nearly an acre of ground, and the
machinery is at present (March, 1889) driven in part by a Pelton water
wheel 11 feet in diameter, and in part by power electrically transmitted
from the Sutro Tunnel level. The mill building stands in a depression
near the head of a small ravine. Such a site was selected in order that
from the time the ore enters the mill its course at each stage necessary
to its complete reduction, shall be downward—that there shall be no
lifting or hoisting of ore or pulp.
The mill stands a little over one hundred yards south of the Chollar
shaft. From the shaft the ore is run in the same cars in which it is
hoisted from the mine directly into the upper story of the mill. It is
there dumped through openings in the floor into the ore bins. Over these
ore bins are placed in a slanting position iron bars three inches apart,
forming screens called “grizzlies.” Through these screens the fine ore
falls into the bins, while the large lumps of rock roll down upon a
floor in front of the rock-breaker, an apparatus that works much on the
same principle as a lemon-squeezer. Between the jaws of this powerful
machine the largest and hardest piece of quartz rock is at once chewed
into fragments sufficiently small to be fed into the batteries, where
the heavy stamps reduce it to pulp. The ore is delivered into the
batteries by self-feeders, which are so regulated as to keep constantly
under the stamps the proper quantity of rock to do well the most work.
At the Chollar (or Nevada) Mill there are sixty stamps,—twelve batteries
of five stamps each. Each stamp weighs about eight hundred pounds. On
the end of each stamp is a heavy head or block of iron or steel called a
“shoe,” and in the bottom of the mortar (a long iron box in which the
stamps of each battery work) is a similar block of iron called a “die,”
upon which the shoe of the stamp strikes when it pulls. It is between
these two blocks of steel that the quartz is crushed.
A small stream of water flows into each battery, and as the ore is
reduced to a powder the water floats it out through the fine screens
that are fitted into the face of each mortar. The pulverized ore and
water, on passing through the screens, falls into a small trough, or
sluice, which carries the muddy mixture down to the settling tanks, on a
floor below, in the amalgamating room. In the tanks the crushed ore
settles and the water runs off. From the tanks the pulverized ore, which
resembles thin mortar, is shoveled out upon the floor alongside the
amalgamating pans, into which it is shoveled whenever they are to
receive a fresh charge of ore.
The pans are of iron and each holds a “charge” of about 3,000 pounds of
the mortar-like pulp. In the bottom of each pan are thick plates of
chilled iron or steel called “dies,” while revolving upon these are
other heavy pieces of steel, called “shoes” or mullers. In the pans the
pulverized ore is ground till it is much finer than when it passed
through the screens of the battery.
When a pan has received its charge of pulverized ore (“pulp”) a small
amount of water is added to render it sufficiently thin to be readily
stirred by the mullers. The pans have tight covers and double bottoms.
The double bottoms are steam chambers by means of which the pulp in the
pan is kept hot while it is ground and agitated. After a charge has been
ground about two hours, some 300 pounds of quicksilver are added (for
3,000 pounds of pulp), also a certain quantity of salt and sulphate of
copper; and sometimes soda or caustic potash and other chemicals, if
thought necessary, when the agitation in the pan is continued two hours
longer. The time of working in the pan varies from three to five hours.
The Chollar Mill has thirty of these pans. On a platform below that on
which stand the pans are fifteen settlers. These are about twice the
size of the pans. At the end of three or five hours each settler has
drawn off into it the contents of two pans. In the settler the pulp,
quicksilver, and amalgam are kept in motion for about two hours. During
this time water is let in and the pulp made very thin. The quicksilver
and amalgam settle to the bottom of the “settler,” and are drawn off
through a pipe and pass into a strainer—a strong canvas bag. There is an
iron box around each strainer, and this is kept locked.
It is in the pan that amalgamation takes place. There the sulphuret and
chloride of silver is changed to the metallic form by the chemical
action of the sulphate of copper (bluestone) and salt, and when it takes
the metallic form it at once unites with quicksilver. The gold contained
in the ore (generally one-third of its whole value) being always in the
metallic form, is amalgamated as soon as it is ground out of its
inclosing shell of quartz, or pyrites of iron.
The thinned pulp—mere muddy water in appearance—on leaving the settlers
passes into large wooden tubs called “agitators,” in which are revolving
rakes. In these tubs is caught some valuable material—principally
amalgam and quicksilver. From the “agitators” the pulp flows out of the
reduction works through a small flume which conducts it to the blanket
sluices, fifty yards away in the open air. The blanket sluices are
broad, shallow flumes in the bottom of which are placed strips of coarse
woolen blanketing. In passing over these blankets the pulp deposits
pulverized iron pyrites containing gold, some fine particles of amalgam,
and quicksilver; also such silver sulphurets as escaped being
amalgamated in the pans. From time to time the blankets are taken out of
the sluices and rinsed in a large tank, in which operation is saved
whatever of value they may have caught.
The amalgam collected in the strainers standing below the settlers is
placed in a press and as much quicksilver as possible pressed out, when
it is placed in retorts, which are heated till all the mercury is driven
off. There then remains behind the silver and gold, in a dull,
rough-looking mass. This “crude bullion” is then broken up and placed in
the melting pots, to be made into “bricks” and assayed. The bars or
bricks made weigh about 100 pounds each. From the top and bottom of each
pot or crucible of molten gold and silver is taken a small quantity of
the fluid metals from which assays are made to determine the value of
the bars. About thirty per cent of the value of the Comstock bullion
bars is in gold, though it has at times run up to fifty per cent in some
mines, and as low as ten per cent in others.
Though the Nevada Mill is in part driven by water, half the power used
is electrically transmitted from six forty-inch Pelton water wheels set
up in a large chamber excavated on the Sutro Tunnel level of the Chollar
Mine, 1,630 feet below the surface. These small Pelton wheels drive six
Brush dynamos, which generate the current that passes over the copper
wires to the electric motors in the mill. The electric apparatus
transmits to the main driving shaft of the mill about sixty-five per
cent of the power developed by the Pelton wheels. Each Pelton wheel
drives a dynamo, and one, two, four or all the dynamos may be run at the
same time, just as may be required, each Pelton and dynamo being
independent of the others.
After the water is used on the large-surface Pelton wheel in the mill it
is caught up and by means of a small flume is conducted to the shaft of
the Chollar Mine, near at hand, down which two large iron pipes carry it
to the six small Peltons below. By thus twice using the same water a
saving of one-half is made. The pressure on the lower Pelton wheels is
immense. Never before has any water wheel been operated under a vertical
pressure of 1,630 feet.
The Nevada Mill was built to work the ores of the Hale and Norcross,
Chollar and Potosi Mines. It is one of the most substantial mills in the
country, and no mill in the State is better arranged. It is lighted with
electricity, and the grounds in front are illuminated by means of an arc
light on a tall mast.
The Two California Mills.
The California stamp and pan-mills in Virginia City reduce the ores of
the Consolidated California and Virginia Mine. The stamp-mill is
situated immediately east of the C and C shaft of the mine. It contains
eighty stamps. The ore crushed in this mill is amalgamated in the
pan-mill, which stands about 1,500 feet further east. The crushed ore is
conducted from the stamp-mill to the pan-mill through an iron pipe four
inches in diameter. The process of amalgamation is much the same as at
the Chollar Mill, except that the pulp goes directly into the
amalgamating pans instead of being first received in settling tanks. It
flows from pan to pan—the outflow of the first pan passing into the
second through a pipe, thence into a third, and so on and from settler
to settler, being in all about three hours in passing through the
series. This is called the Boss Continuous Process. It is in use in no
other mill on the Comstock, as yet. In connection with the Rae
electrical process of amalgamation (in which a current of electricity is
passed through the settlers) it is found to work satisfactorily. The
electric current prevents loss of “floured” quicksilver. Both mills are
driven by Pelton water wheels. A single Pelton wheel eleven feet in
diameter, placed on the surface, drives the eighty stamps of the
battery-mill, and also twelve Boss grinding pans. The water used on the
surface Pelton is caught up and conducted to the C and C shaft, where it
is used on a series of Pelton wheels of the same size. These wheels are
placed in chambers made for their reception 500 feet apart from the top
of the shaft down to the Sutro Tunnel level (there 1,500 feet), and by
means of steel wire cables, used as belts, the power of all the lower
wheels is brought to a main driving shaft on the surface. The whole
power is then transmitted to the pan-mill (about 1,600 feet) by means of
steel wire cables passing over pulleys placed on a series of tall wooden
towers. The cables pass over a considerable depression between the top
of the C and C shaft and the pan-mill; three high towers are required in
the middle portion.
River and Canyon Mills.
The Mexican Mill, on the Carson River, contains forty-four stamps and a
corresponding number of pans, settlers, and other amalgamating
machinery. The Morgan Mill has forty stamps. It works ore from the
Consolidated California and Virginia Mine. The Brunswick Mill contains
seventy-six stamps, the Vivian sixteen, Santiago thirty-eight, and
Eureka sixty. All these mills are about and below Empire City, and all
work Comstock ores. The Eureka Mill is run on ore from the Consolidated
California and Virginia. The Rock Point Mill (thirty stamps), at Dayton,
and the Douglas Mill (ten stamps), in Lower Gold Hill, also work
Comstock ores.
At and about Silver City are two or three small mills that work the ores
of mines in that neighborhood, and on the Carson River are the Douglas
and Woodworth Mills, which work tailings.
On Six-mile Canyon, below Virginia City to the east, are several small
water mills having an aggregate of about thirty stamps. These work ores
from the mines on the canyon and in Flowery District. On the canyon are
also one or two small mills that work tailings and the concentrations
from blanket sluices.
The Alta Mining Company has a ten-stamp mill, with concentrators,
immediately adjoining the hoisting works at their mine. The Justice
Company have a new ten-stamp mill near their mine.
Owing to the fact that many mines are now at the same time producing
large quantities of ore, a lack of milling facilities is being felt. To
meet this demand the Nevada Mill has been enlarged one-third, and the
capacity of other mills will be increased, and perhaps some new mills
will be erected. Processes by means of which low-grade ores may be
profitably worked will no doubt yet be invented or discovered, which
will cause many new works to be erected either on the Carson River or in
the neighborhood of the mines producing large quantities of such ores.