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.