Gold and Silver from Ores.
I visited a huge smeltery not long ago, and saw how gold and silver were separated from their ores. The lead ore, or galena, which contains also gold, silver, and copper, is brought from Utah. The average yield of silver of the ore used here is about one hundred ounces to the ton. The amount of gold and silver in the ore is determined in the assaying room in this manner:
A piece of the silver-bearing lead is carefully weighed in a very delicate balance, and is then placed in a little cup of bone ash, called a cupel. Then the cupel is put into a very hot furnace so arranged that a current of air passes over it. The air oxidizes the melted base metals, but the gold and silver are not affected. The cupel has the remarkable power of absorbing the oxides of metal, and so in an hour or so there is nothing left in it save a little bead of silver and gold. This bead is then weighed, and in this way it is known what proportion of gold and silver there is in the ore.
To extract the metals, the ore is mixed with limestone and coal, and is thrown into a blast-furnace, which resembles an inverted cone. A fire is started in the bottom and a blast of air is forced through the pipes into the furnace. When the metal has been melted from the ore the furnace is tapped at the bottom, and the metal, consisting of gold, silver, copper, and lead, runs out into large pots. It is then run into moulds. This metal is called "bullion."
The next process is to separate the lead from the other metals. The bullion is melted in a large deep basin and molten zinc is added. The zinc forms an alloy with the gold, silver, and copper, which is lighter than the lead, and therefore floats on the surface. Then this alloy is skimmed off and taken to another part of the works, where it is placed in furnaces and the zinc burned out. After all the zinc has been gotten rid of the metal is taken to a large room which contains a row of small furnaces. Inside of these furnaces are shallow cupels over which a current of air passes. After the metal has been melted in these cupels it is run out into moulds, which shape the metal into plates about twenty inches long and ten in width.
The metal of these plates consists of gold and silver, which still have to be separated. The plates are hung in gauze bags and put in strong nitric acid. This acid dissolves the silver, but does not affect the gold, which drops down into the bags and is caught there. About three inches distant from the sack containing the gold and silver plate is a very thin plate of silver. This plate and the one in the sack are connected to a dynamo. The current of electricity causes the dissolved silver to deposit itself on the plate. After all the silver has been collected it is cast into blocks weighing one thousand ounces each. The gold is likewise cast into blocks.
I saw about $100,000 worth of silver in the vaults and in the works. There are other methods of separating these metals, but I think this is the most common way. Some ores are more easily worked than others.
Homer L. Stewart, R.T.F.
Pittsburg, Pa.