Imágenes de páginas


LEAD and zinc are closely associated in nature, their ores commonly occurring in conjunction, especially their sulphide ores. There is less association of their oxidized, or carbonate, ores, since in the process of oxidation the original zinc sulphide is converted first into sulphate, which is very soluble and may therefore be leached away by some action of nature; but in a large number of cases, as for example at Leadville, Colo., the occurrence of pure lead-carbonate ores above the water level is followed by the occurrence of mixed lead and zinc sulphides below the water level. Both lead-sulphide ore and zinc-sulphide ore occur alone, but the occurrence in association is more common, and the importance of the joint consideration of the two metals from the industrial standpoint increases as the oxidized ores are exhausted and the smelters have to fall back on the sulphide ores, which have already become by far the more important, in the metallurgy of both lead and zinc, all over the world.

It will be observed from a study of the literature of economic geology that the deposits of both lead and zinc ore, whether in association or alone, form most kindly in connection with a dolomite or limestone country rock. Most of the great lead and zinc ore deposits of the world occur in formations of those species of rock.

In their metallurgy there is also an intimate association of the two metals. In the first treatment of the ores, by jigging or other methods of separation, there is generally obtained a galena concentrate, which goes to the lead smelter for the extraction of its lead content, and a blende concentrate, which goes to the zinc smelter for the extraction of its zinc With few exceptions, the crude ore has first to be separated in some such manner. The limitations of mechanical practice do not, however, permit a clean separation of the minerals to be made, save in rare instances, and the lead concentrate consequently almost always contains some zinc, and vice versa the zinc concentrate contains some lead. The presence of zinc is objectionable to the lead smelter and the presence of lead is objectionable to the zinc smelter, but in neither case does the presence of the other metal in limited proportion prevent successful smelting. But while the lead smelter is bound inevitably to lose all the zinc contained in the lead ore that he smelts, the zinc smelter may recover a large proportion of the lead in the zinc ore, after the zinc

itself has been extracted. Thus a very large proportion of the pig lead produced in Belgium is recovered from zinc-ore residues, and this practice has been inaugurated in the United States and is without doubt destined to increase in importance.

Finally, the chief use for pig lead is in the manufacture of whitelead pigment, and one of the large uses of zinc is in the manufacture of white pigment, which in the trade is a direct competitor of white lead. But white lead and zinc oxide are to a large extent used in a pigment compounded of both, so that it is conceivable that the lead and zinc. which existed originally in the same ore deposit, and were mined and separated by a common operation, might be smelted by a combination process and finally after conversion into finished products might be mixed again for use in the arts and be subject to common dissipation under the action of weather and time.

Up to 1901 it might have been possible to consider independently the lead and zinc industries of the United States, but from the economical standpoint it will never more be possible to do so.

The existence of lead ore in the territory that now comprises the United States became known in the earliest explorations, and mining was quickly begun on such deposits as appeared workable. In southeastern Missouri and southwestern Wisconsin, where the discovery of workable deposits antedates all other important discoveries of lead in the United States, the operation of the mines has continued uninterruptedly up to the present time, at which Missouri gives promise of producing more lead than in all its past history, while a rejuvenation of mining in Wisconsin is beginning. There is in these two regions an antiquity of mining and continuity of record that is comparable with the history of some of the famous districts of Europe.

The history of lead mining in the United States is capable of division into epochs, marked by important discoveries, or industrial developments of determining effect. It is a coincidence that these have occurred to a considerable extent at decennial periods. From 1720 to 1820 the total production of lead was small compared with the annual production at the present time. The entire output during the 100 years was only about one-seventh the output of a single year at the beginning of the twentieth century. It was derived almost wholly from southeastern Missouri, territory which did not become a part of the United States until the Louisiana purchase in 1803. In 1821 attention began to be attracted to the lead mines of Wisconsin, discovered many years previously, possibly as early as 1634, and certainly as early as 1658, but unattended to because of the Indian occupancy of the region. A few years later mining was in full swing, and by 1828 the output had become important. The 50 years from 1821 to 1870 may be called the Wis

« AnteriorContinuar »