METALLURGY. For metallurgical purposes the chief ore of lead is galena. Next come the carbonate and the sulphate, the former of which is rarely smelted by itself and the latter very exceptionally treated alone. The following de scription, therefore. refers entirely to the reduc tion of galena. There are three general proc esses for reducing galena ore which are known as the air reduction process, the roasting and carbon reduction process, and the precipitation or iron reduction process. All of these processes require a subsequent refining process for the production of commercial lead pig.
In the air reduction process the galena is first roasted to such a degree that a mixture of lead sulphide, oxide, and sulphate is produced con taining the three compounds in definite propor tions, and then the temperature is increased, causing a reaction between the elements named, which produces metallic lead and sulphur dioxide. The operation is performed in rever beratory furnaces or in hearths, the former being the more common method. When the process is conducted in a reverberatory furnace the reduc tion period follows immediately after the process of roasting; in hearth, the roasting and reduc tion proceed simultaneously. ln the rever beratory furnace process it charge of ore weigh ing from one to four tons is placed in the furnace and subjected to roasting at a eonstantly increasing temperature for from two to four hours. The temperature is kept down by keeping the doors open, and the charge is con stoutly stirred. The furnace doors are then closed and the fire urged fur a period varying from an hour in some forms of the process to seven hours in other forms. During this opera tion metallic lead is formed and runs into the furnace slump, whence it is drawn at intervals into suitable ladles. The temperature is then lowered and another period of roasting takes place; this is followed by a second increase of temperature, during which more metallic lead is obtained. These alternate periods of roasting and reaction are continued until the ore has given up all its lead.
In the hearth process the operations are in outline as follows: On the hearth, which has been previously almost tilled with lead, some coal is strewn, and when the fuel is well lighted and glowing, the residue from a previous opera tion is placed on it. As soon as the ore begins to separate out a quantity of ore mixed usually with a little lime is thrown on the glowing mass and covered with a small portion of fuel. After ten or fifteen minutes the under side of this mass is stirred together and is then broken up and withdrawn onto the workstone, the top portion of the mass now sinking down in the hearth. On the wo•kstone the slag is separated from the incompletely decomposed ore, and the latter, mixed with a fresh quantity of ore and a little slacked lime, is put back into the hearth, the separated slag being collected and worked up on the slag-hearth. After five or ten minutes the slag on the hearth is again broken up, the lower and partially smelted part withdrawn from the hearth and treated as before.
A constant repetition of this operation constitutes the hearth The separated lead trickles through the fuel into the hearth, which. when full, overflows into suitable ladles.
In the roasting and carbon reduction process the we is first roasted in heaps or in roasting, furnaces of various forms, with the addition of quartz to bring the lead in the ore into the form of oxide and silicate. The roasted ore is next smelted in blast-furnaces, alternate layers of fuel and ore being charged at the top and a blast of air being introdueed at the hottom. The products of the blast-furnace are metallic lead, lead matte, slag, and sometimes speiss. The molten lead is ready for refining. but the matte and speiss must he subjected to further ,roce--ss to secure the metallic lead contained. In the ease of matte this process consists first of roasting and second of smelting. exactly as in the case of the original ore. The roasting and carbon reduction process is the one most widely used for smelting lead from galena.
In the iron reduction process the galena is smelted in reverberatory furnaces or. more com monly, blast-furnaces, with the addition of iron ore or metallic iron to produee a matte. The matte is then roasted and smelted to obtain the metallic lead. This process is seldom used alone, but is practiced to a considerable extent in combination with other processes.
The preceding processes are all employed with galena ores. Lead carbonate is smelted in blast furnaces with suitable fluxes—limestone and slag from previous operations—the metallic lead draining into suitable slumps at the bottom of the furnace. Lead sulphate is usually smelted in connection with other 'dies. generally galena, but it may be smelted alone by one of several methods.
The lead obtained by any of the processes described contains impurities such as copper. arsenic. antimony. zinc, iron. bismuth, tin. and sulphur, and their removal where they exceed a certain proportion is necessary to produce marketable lead. Refining processes resolve themselves into the separation of copper by liquation or by means of zinc. and the extrac tion of the other metals with the exception of bismuth and the precious metals by melting in an oxidizing atmosphere. In liquation the copper rises to the surface of the molten lead in the form of scum which is skimmed off. and in the zinc process it combines with the zinc to form zinc scum. which is similarly removed. Oxida tion of the other metals is brought about by melting and heating the lead. when they pass off one after another as the proper temperatures are reached. The melting is usually done in pots. but hearths and reverberatory furnaces are also employed. Lead containing silver may be re fined by electrolysis. After being refined the molten lead is cast in molds into pigs and is then ready for the market. For a concise de scription of the metallurgy of lead. consult Schnable, Handbook of Metallurgy (London, I S9'4 )