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.
CHAPTER XLVII.
ASSAYS OF THE SILVER BULLION.
The water and pulp discharged from the settlers runs through sluices to the lowest part of the building, where, some eight or ten feet below the level of the floor of the amalgamating-room, stand the agitators, four in number. These are huge tubs, having in them revolving rakes or “stirrers,” and here is caught whatever valuable matter may have passed through the settlers.
Twice in twenty-four hours, the heavy matter collected in the bottom of the agitators is cleaned out and placed in four small pans and two settlers that stand in the same room to be re-worked. Finally, the pulp leaves the agitators and, carried by a quantity of water to float it, passes out of the mill in a trough or flume through which it flows eastward to a considerable distance from the mill, when it reaches what are called the “blanket sluices,” the working of which will be described further on. In speaking of the pans and settlers, I have described but one row or set. The two rows, one on the north and the other on the south side of the large room, are exactly alike. Each row of pans has its row of settling tanks, settlers and amalgam strainers. To these strainers, in which we left the amalgam and quicksilver, a few minutes since, we now return.