Page:The New International Encyclopædia 1st ed. v. 12.djvu/53

LEAD. •41 LEAD. bonate, the sulphate, the pliosphate, a chloroar- seiiate, as well as nunieious compounds with rare elements, such as chromium, selenium, tellurium, vanadium, etc. Lead Ores. The principal ore of lead is galena (PbS), which contains 8(i.G per cent, of metal. The sulphate (anglesite). the carbonate (cerusite), and the phospliate (pyromorphite) are occasionally found in sullicient ijuantities to warrant exploitation: while the arsenate, chro- niate, vanadate, molybdate, and oxide have little metallurgical importance. Galena always con- tains some silver and usually a little arsenic, antimony, copper, zinc, and gold. When the content of silver readies several ounces a ton, the ore is known as argentiferous galena, and is treated for the separation of the metals. Galena is a widely disseminated mineral, although it rarely occurs in large deposits. Along the At- lantic border of the United States there are numerous localities where it is found in veins that cut through the Arclntan and Paleozoic rocks, the gangue material being quartz or calcite. These deposits have not been worked, however, for many years, owing to the great abundance of rich ores in the Western States. One of the most productive mining regions is in Southeastern ilissouri. The ore is dissemi- nated in limestone through a thickness of about 200 feet^ and it lias been taken out in enormous quantities from tlie workings at iline La ilotte. Doe Run. and Bonne Terre. Galena associated with zinc lilende is found in Illinois, Wisconsin, and Iowa in proximity to the Jlississippi' River, and also in southwestern Missouri in .Jasper and Newton counties, and across the Kansas border in Cherokee County. The argentiferous lead ores of the Rocky ^Mountain States are the principal sources of supply at the present time. The ore bodies are found in carboniferous lime- stone, for the most part along the contact with sheets of porphyiy : they yield hard and soft ores containing cerusite and silver chloride. In the deeper workings the oxidized ores give way to the unchanged sulphides. Lead-silver mines are operated at several localities in Utah. Large quantities of lead in crude form are imported into the L'nited Slates from Mexico and British Columbia and refined by Western smelters. MET.iLLiRGY. For metallurgical purposes the chief ore of lead is galena. Next come the carbonate and the sulphate, the former of which i< 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 larbon reduction process, and the precipitation ir 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 r'lnsted to such a degree that a mixture of lead -iilphide, oxide, and sulphate is produced con- 1;iining the three compounds in definite propor- tions, and then the temperature is increased, I causing a reaction between the elements named, I which produces metallic lead and sulphur dioxide. The operation is performed in rever- li'ititory furnaces or in hearths, the former being 1 1 11' more common method. When the process is conducted in a reverberatorv furnace the reduc- tion period follows immediately after the process of roasting; in hearths the roasting and reduc- tion proceed simultaneously. In the rever- beratory furnace process a eliarge of ore weigh- ing from one to four tons is placed in the furnace and subjected to roasting at a constantly increasing temperature for from two to four hours. The temiierature is kejit down by keeping the furnace doors open, and the charge is con- stantly stirred. The furnace doors are then closed and the fire urged for 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 slumps, 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 filled with lead, some coal is strewn, and when tlie 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 mi.xed 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 jnass is stirred together and is then broken up and withdrawn onto the workstone, the to]) portion of the mass now sinking down in the hearth. On the workstone 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 process. 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 ore 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 introduced at the bottom. 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 be subjected to further process to secure the metallic lead contained. In the case 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 furmiccs or. more com- monly, blast-furnaces, with the addition of iron ore or metallic iron to produce a matte. The matte is then roasted and smelted to obtain the metallic lead. This process is seldom used