EXTRACTING SILVER FROM THE ORE.
Having shown the reader what is to be seen in the underground regions of
the mines, I shall now proceed to show him what is to be seen in a
quartz-mill, explaining the use of the machinery and various processes
for the extraction of the silver from the ore. I shall begin with the
ore as it comes from the mine, and follow it through the reduction-works
until it makes its appearance in the shape of silver bars, stamped with
their value, and ready for the mint or the market.
The mills in which the ores of the Comstock lode are reduced, are all
built on the same general plan. When the tourist has visited and
examined one mill, he has seen them all, both great and small, so far as
regards the processes in use for the reduction of the ore. Some mills
are more conveniently arranged than others, however, and while in some
machinery is used which is somewhat behind the age, in others will be
found in operation in every department machinery of the latest and most
approved pattern.
The model mill of the State, and of the world, for the reduction of
silver ore, is the new 60-stamp mill of the Consolidated Virginia Mining
Company. In this mill is to be found all that is valuable in any mill,
and much in the way of machinery that can be seen in no other works of
the kind.
[Illustration: LOADING SILVER ORE—CONSOLIDATED VIRGINIA MILLS.]
In describing a quartz-mill, and the processes used in working the ores
of the Comstock mines, I shall, therefore, select the Consolidated
Virginia reduction-works as those through which to conduct the reader.
The Consolidated Virginia mill stands about 200 feet north-east of the
company’s main shaft and hoisting-works. The ground was well chosen,
there being a considerable incline toward the east, which allowed of a
proper and regular descent from the battery-room on the west to the room
containing the agitators on the east, so that the course of everything
is downward, from the time of dumping the ore into the chutes at the top
of the mill. The ground was graded out in regular terraces of the proper
size for the several departments, as the initial step, and in their
proper order were reared upon these, foundations for the various kinds
of machinery, and the whole covered by one immense building or series of
buildings, principally under one roof—a vast aggregation of buildings
and machinery.
The battery-room, with ore-bin, etc., is situated on the west side of
the mill, and is 100 feet in length by 58 feet in width. Immediately
adjoining this, on the east, on a terrace a few feet lower, is the
amalgamating-room, containing the pans, settlers, and other amalgamating
apparatus. This room is 120 feet in length by 92 feet in depth. East of
this, and a few feet lower down, is the room containing the agitators
and other apparatus connected therewith. This room is 92 feet in length
by 20 feet in width. North of the amalgamating-room is the engine-room,
containing the engine and boilers. This room is 92 feet long by 58 feet
in width. Near the mill stands a handsome office, 20×30 feet in size;
and to the eastward, and distant from the mill some 30 feet, is the
retort-house, built of brick, and 20×60 feet in size.
To drive the whole of the machinery of the works there is a compound
condensing-engine of 600-horse power. This engine has two cylinders, the
first 24×48 inches, and the second 48×48 inches in size. The steam is
admitted to the first or “initial cylinder,” where it is cut off at half
stroke. It then passes into the second or “expansion cylinder,” which,
being twice the size of the first and having four times its capacity in
cubical contents, gives an expansion of eight bulks—twice in the first
cylinder, and four times in the second. After the steam has left the
expansion cylinder, instead of exhausting in the open air it exhausts
into a condenser, where it gains an additional power equal to the
atmospheric pressure at the altitude of Virginia. The main shaft from
this engine is 14 inches in diameter, and weighs 15,000 pounds. On this
shaft is a fly-wheel (which is also a band-wheel and carries the large
belt by which the batteries are driven) 18 feet in diameter and weighing
16½ tons. On the extreme end of the main driving shaft is coupled a
shaft 11 inches in diameter, which extends into the amalgamating-room
and drives the pans and settlers—indeed, all the machinery except that
connected with the batteries. The whole weight of the engine is about 50
tons, and it stands on a foundation of 450 cubic yards of masonry, laid
in cement, the weight of which is over 600 tons. There are in this room
four pair of boilers, eight in all, each of which is 54 inches in
diameter, and 16 feet in length. All of these boilers can be used
simultaneously, or each pair can be run separately—just as may be
required. From the floor of the engine-room to the ridge of the roof the
distance is 50 feet. The west side of this, and of some of the adjoining
rooms, is formed by a stone wall 22 feet in height. In these walls there
are in all, 4,000 perches of mason-work—all trachyte rock. The
smoke-stacks of the boilers are four in number, and each is 42 inches in
diameter and 90 feet in height. In this room are two large steam-pumps
for use in feeding the boilers, or to be used for fighting fire, if need
be; each being supplied with hose of sufficient length to reach to any
part of the building.
About 28 cords of wood are used per day—10,080 per annum. This wood is
brought to the mill from a side-track of the Virginia and Truckee
Railroad, on a truck which holds exactly one cord. Thus is the wood
measured as it is delivered. The truck dumps the wood into a chute,
which carries it down into the boiler-room, and it is landed just in
front of the furnaces, where it is wanted.
We will now return to the west side of the mill and ascend to its
extreme top, even above the roof. Here, above the roof, comes in a large
car-track, leading directly from the main shaft of the hoisting-works at
the mine. This track is 278 feet in length, and is housed in for its
entire length. It is handsomely finished off, contains windows its whole
length, is painted a light brown color, and strikingly resembles a
rope-walk.
When the cages bring to the top of the shaft the cars loaded with ore, a
carman is standing ready, who takes the car from the cage and pushes it
before him over an iron track to the chutes which lead down through the
roof of the mill into the huge ore-bin below. This car-track, and the
long building covering it, are supported upon a strong trestle-work
constructed of large square timbers, and rising forty-four feet above
the surface of the ground in the highest part. To keep the stamps
supplied with ore requires one car-load to be sent out from the shaft
every five minutes during the day and night. Although the cars were at
first pushed out over the track by hand, they are now made up into
trains of ten, and are hauled by a mule from the hoisting-works to the
mill.
The ore, on being dumped into the chutes at the top of the mill,
descends to the centre, from each side. The chutes have in their bottoms
what are called “grizzlies”—iron bars placed three inches apart so as to
form a screen—through which the fine ore drops into the bin below, while
the coarse rock rolls on down and is dumped on a floor above the
ore-bin, and about its centre, where stands the rock-breaker.
The rock-breaker is a heavy piece of machinery, which in appearance, and
the principle upon which it works, not a little resembles a huge
nut-cracker or lemon-squeezer. It is the same kind of machine that is
used in some cities for chewing up rock for macadamizing streets, and
which is known as a “masticator.”
The coarse rock being crushed in the rock-breaker is carried into the
ore-bin by a chute. In the main chutes above are what are called
distributing chutes, which are chutes that carry the descending ore far
away from the centre of the bin. But for this arrangement, all of the
ore would fall in the middle of the bin, which is 110 feet in length.
In the battery-room are ranged in a row, north and south through the
building, six batteries of ten stamps each, or sixty stamps in all. Each
stamp weighs 800 pounds. Each set of ten stamps works independently of
each other set, and can be stopped and started at will by simply moving
a sort of brake or clutch. The whole of the stamps and the apparatus
connected therewith, are driven by a belt from the main fly and
band-wheel (mentioned above), which belt is 24 inches in width and 160
feet in length. This runs the counter-shaft in front of the batteries,
and from the pullies on this counter-shaft there are belts 14 inches in
width and 60 feet in length, which run each battery of ten stamps. The
main belt, which drives the whole of this machinery, runs at the rate of
3,600 feet per minute.
From the ore-bin the ore descends into the Tulloch self-feeders, one of
which machines is required for every five stamps, or twelve in all.
These do the whole work of feeding. The ore is not touched by anyone
after it falls into the bin. Two men are able to keep watch over all the
feeders supplying ore to the whole sixty stamps. The feeder is the
invention of James Tulloch, of California, and is a very valuable
labor-saving apparatus. The feeders are self-regulating, the motion of
the stamps in dropping, operating them. When there is too much ore in
the battery, the tappet of the stamp does not fall sufficiently low to
strike the end of the rod attached to the feed-table, and no more rock
enters the battery for a time; but as the rock is worked out, the feeder
again begins to operate. In most of the mills the ore is still fed into
the batteries, with shovels, by men known as “feeders.” When the feeding
is done by hand, the amount of ore reduced in a given time, depends much
on the men who do the work. They must put under the stamps all the ore
they can crush, and no more. This must be done constantly throughout the
twenty-hours for weeks and months.
In the Consolidated Virginia mill, the mortars—the huge iron boxes in
which the stamps work—do not discharge the pulp or pulverized ore in
front, as is usual, but at one side. This gives free access to the
mortars in front for the purpose of putting in new shoes and dies. The
“shoes” are the heavy blocks of iron or steel fastened to the lower end
of the stamp. It is the shoes that fall upon and crush the ore when the
stamp is dropped by the cam which raises it. The “dies” are much the
same in shape and size as the shoes, and are fitted into the bottom of
the mortar in such a position that one is exactly under the point where
the shoe of each stamp strikes. Thus it is between the “shoes” and
“dies” that the rock is pulverized.
[Illustration: FIRST QUARTZ MILL IN NEVADA.]
[Illustration: QUARTZ-MILL—AMALGAMATING ROOM.]
A small stream of water is constantly running into the battery among the
ore, which water, being strongly churned and agitated takes up and
floats all of the finer particles of ore. Across the face of the mortar,
just in front of the dies, are the screens, made of the best Russian
sheet-iron, punched full of small holes. Through these holes the water
and the finely powdered ore pass into a sluice or trough running to the
settling-tanks in the amalgamating-room, where the ore, now in the shape
of fine sand, is deposited, to be finally shovelled out and placed in
the amalgamating-pans. The finer the screens the smaller the quantity of
ore that can be put through a battery in a given time.
The roar of Niagara is as a faint murmur compared with the deafening
noise of sixty stamps, all in full operation. In the battery-room, and
indeed throughout the mill, the noise is such that it is almost
impossible to converse. Every word must be shouted into your ear at the
top of the speaker’s voice, and in a tone that would be audible at the
distance of a mile in the open air. There is little talking done in the
battery-room; except when ladies visit the works; then you can see that
their lips continue to move, and the presumption is that they are
talking right straight along.
Just in front of the battery-room, but having its floor some feet lower,
is the amalgamating or pan-room, 92×120 feet in size. Into this room
comes the pulp as it runs from the batteries. The pans stand in two long
lines, running east and west, and back of the lines of pans are the
settling-tanks, while in front of them are ranged the “settlers,” a
large kind of pan into which the pulp passes from the pans proper—the
amalgamating-pans. On each side of the building, over the
settling-tanks, are sluices bringing the pulp (mingled with water) from
the batteries. Each sluice brings the pulp from thirty stamps, and
supplies one row of settling-tanks—there being spouts leading from the
sluice to each tank. There are seventeen of these settling-tanks, and
when the pulp has settled in them till it is of the consistency of thick
mortar, it is shovelled out upon a platform which runs alongside the row
of amalgamating-pans. There are sixteen pans in each row—thirty-two in
all—and each pan is five and one-half feet in diameter, and holds a
charge of 3,000 pounds of this pulp.
In the bottom of the pans are thick plates of cast-iron called “dies,”
while revolving upon these are the mullers, which are furnished with
other thick plates of iron called “shoes.” It amounts to much the same
thing as the shoes and dies in the batteries, except that in the latter
the ore is pulverized by percussion, while in the pans it is done by a
rotary motion—by grinding.
When the charge of pulp has been shovelled into an amalgamating-pan, a
certain quantity of water is added to thin it to the proper consistency
for working, when the mullers are set in motion, and the work of
grinding the ore in the pan begins. The pans have covers and double
bottoms, and when they are at work, steam is not only let into the pulp,
but also underneath, between the two bottoms.
After the pulp has been thus heated and ground for two and a half hours,
there is placed in the pan 300 pounds of quicksilver, and it is run two
and a half hours longer—five hours in all. Besides the quicksilver,
there is put into the pan with the charge a certain quantity of salt and
sulphate of copper; also, when thought necessary, soda and some other
chemicals are added.
The foundation of this method of working silver-ore is the old Mexican
patio process. When Americans came to engage in the working of silver
ores, upon the discovery of the Comstock lode, they found the Mexican
plan of working too slow, and they began to study, in order to make
improvements in it. In the Mexican patio process the pulverized ore is
made up into a thick mortar on a floor of planks or stone (which is the
patio), when salt and sulphate of copper are added and mixed in, and the
pile of mortar is built up in the shape of a mound, and allowed to heat
and sweat.
After a proper time has elapsed the mound is pulled down and spread
about, when quicksilver is sprinkled upon and well worked into the mass,
and it is again made up into a mound-shaped pile, to heat. This pulling
down and building up, spreading about, and airing, is several times
repeated, and the whole operation lasts a number of days, when finally
the mass of mortar is washed and the quicksilver and amalgam secured. By
placing the pulp, or mortar, in large iron pans, heated by steam and
stirred by machinery, we see that the time of bringing the ore to the
metallic state, is reduced from five or six days to as many hours. The
principle involved in the two processes—pan and patio—is essentially the
same.
On a platform below the amalgamating-pans, stand eight settlers, one for
each pair of pans. The settlers are each nine feet in diameter, and five
or six feet in depth. Into the settlers, at the end of five hours, the
contents of the pans—quicksilver and all—are drawn. The pulp,
quicksilver, and the amalgam (silver and quicksilver combined), remain
in the settler about two hours, during which time the quicksilver and
amalgam are drawn off through a pipe, at the bottom of the settler, and
run into strainers, one of which stands in front of each settler, and
all of which are provided with iron covers that are kept locked.
The silver separates from the ore while in the amalgamating-pan, being
changed from the chloride and sulphuret to the metallic form, by the
action of the salt and sulphate of copper. As soon as it has assumed the
metallic form, it unites or amalgamates with the quicksilver, but both
in the pan and in the settler it is still mingled with the earthy matter
of the ore from which it was produced.
It is first seen freed from this gross and earthy matter—pulverized
rock, principally quartz—when it passes from the bottom of the settler
through the iron pipe into the top of the strainer. Then it is mingled
with nothing more base than quicksilver.
The strainers are bags of heavy canvas suspended in strong boxes,
covered, as has been mentioned, with iron lids, somewhat funnel-shaped,
and perforated with holes through which the quicksilver and amalgam may
pass to the straining-bags—where we will leave them for the present.