There are three general methods of extracting copper from its ores. The dry, wet, and electrolytic methods. The wet method is used the least of the three, and consists in placing the ores in an acid solution which dissolves the copper, which is then precipitated to the bottom of the tank by the addition of suitable precipitants.
The dry method is the one generally used for reducing the ore, especially when it is rich in copper. This method consists of two operations; First, roasting the ores; and second, smelting the roasted ore in a blast furnace.
The ore is first "heap roasted" out of doors, the fuel being wood. One cord of wood is all that is necessary to roast 40 tons of ore. The wood is placed so as to form chimneys thru the ore pile. The wood is then fired, and the burning wood releases and sets fire to the sulphur of the ore which burns and again releases more sulphur. In this manner the "roasting pile" will burn for several weeks.
In copper smelting two kinds of furnaces are used, blast furnaces, and reverberatory furnaces. The smelting of copper ore in a blast furnace is the process of reducing the copper from its ores by subjecting the ores to a fierce heat in a cupola. Coke is the fuel used, and the fierce heat is obtained by turning into the cupola a blast of air heated to about 800 degrees Fahrenheit.
The largest blast furnace in this country is at the Washoe Works of the Anaconda mine, in Montana. It is 80 feet long, and has a capacity of 2,700 tons of ore a day.
The reverberatory furnace is one in which the heat from the fuel is reflected back again to the ore, giving a steady but less fierce heat. The reverberatory furnace is used on the sulphide ores in preference to the blast furnace, as sulphur contained in the ore assists in the reduction of the ore. The first reverberatory furnace for copper smelting was built in 1765 in Yorkshire, England. The largest reverberatory furnace is at the Anaconda mine in Montana. It is 119 feet long, and has a daily capacity of 300 tons of ore.
The Bessemer converter is a type of blast furnace that is also used in smelting the ores of copper. These converters are cylindrical shells, made of boiler plate steel, about 4 feet in diameter and 10 feet high. These shells are mounted on a trunnion, with a tilting device for emptying the charge. Air is blown thru the melting ore at a pressure of 14 pounds to the square inch. The condition of the ore is judged by the color of the flames issuing from the top of the converter. Fifteen tons of ore can be converted to a metallic state in about one hour. The resulting copper is known as blister copper, and is used as "anodes" for the electrolytic method.
When especially pure copper is desired, or when the ore is known to contain a considerable percentage of gold and silver (which is often the case), the electrolytic method is used. This method consists of attaching a thick plate of "blister" copper (called the "anode"), weighing about 200 pounds, to the positive pole of a dynamo, and a thin sheet of copper, called the "cathode," to the negative pole. These plates are immersed in an acid solution, and an electric current is passed from the "anode" thru the solution to the " cathode." This results in the dissolving of the anode, which is deposited upon the cathode, the gold and silver and other impurities falling to the bottom of the solution. The copper deposited upon the cathode is pure copper and the precious metals at the bottom of the solution are recovered by the use of precipitants and mercury. The metals that are precipitated to the bottom of the solution usually consist of about 40 per cent silver, 25 per cent copper, 2 per cent gold, 10 per cent arsenic, the balance consisting of lead and other impurities.
The acid solution used in this process consists of: sulphuric acid, 10 per cent; Milestone, 15 per cent; water, 75 per cent, and a very small percentage of sodium chloride for the purpose of precipitating the silver. The solution is contained in large wooden tanks lined inside with lead and painted with tar.
Electrolytic refining costs about $12.00 per ton, and the bulk of the world's copper is treated by this method. The copper thus produced averages 99.93 per cent pure; it is also the best conductor of electricity, indicating a conductivity of 104 per cent.