has for some time been conducting investigations in the Wisconsin and the Missouri zinc fields in order to ascertain the causes of the FIGURE 1.-Present source of zine, and also ores not utilized. losses in milling zinc ores in those States, and, if possible, to devise improvements in the processes used. a Wright, C. A., Mining and milling of lead and zinc ores in the Wisconsin district, Wisconsin: Tech. Paper 95, Bureau of Mines, 1915, 39 pp. ▸ Wright, C. A., and Buehler, H. A., Mining and milling of lead and zinc ores in the Missouri-KansasOklahoma zinc district, Bull. 154, Bureau of Mines, 1918, 134 pp. Dump. Wright gives the following interesting data on the efficiency of hydromechanical concentration in the Joplin district. The ores require the simplest form of hydromechanical concentration. Efficiency of concentrating methods in the Joplin district. Quantity of ore through mill, tons.. Quantity of zinc concentrate produced, tons... Quantity of blende (ZnS, 67 per cent Zn) in concentrate, tons. Assay of tailing from mill (0.96 per cent Zn) per cent ZnS.... Total quantity of blende (ZnS) in ore through mill, tons.. 309.70 9.235 59. 8. 132 300. 465 1.43 4. 296 12. 428 65.43 The average loss, therefore, in the concentration of zinc in the Joplin district is 35 to 40 per cent. There are heavy losses in smelting. The total loss reaches nearly 50 per cent. ELECTROMECHANICAL PROCESSES. MAGNETIC SEPARATORS. A magnetic separator contains a set of magnets by which paramagnetic or weakly magnetic minerals brought into the magnetio field are influenced in such a manner as to effect the desired separation. For example, a mixed sulphide ore crushed to the required degree of fineness and passed over a wet-concentrating table yields a galena-pyrite product and a pyrite-blende product. If the latter product be passed over a magnetic concentrator, such as the Wetherill, two products are obtained: (1) A blende product assaying about 50 per cent zinc, 10 to 12 per cent iron, and 1 per cent lead-which is sold to the zinc smelters; and (2), a pyrite product, containing some lead and about 5 to 7 per cent zinc, which is mixed with the galena-pyrite product from the concentrating tables and is sold to the lead smelters, for whom it is a desirable ore. When this ore is smelted in the lead blast furnace, the zinc is lost in the slag and fume. ELECTROSTATIC SEPARATORS. Electrostatic separators are based on the difference in electrical conductivity of different minerals and on the attraction of bodies having opposite electrical charges. Most of the gangue minerals and some of the metallic sulphides, for example, blende, are poor conductors, whereas many metallic sulphides and other minerals. are good conductors, as pointed out by Richards: • Wright, C. A., Mining and treatment of lead and zinc ores in the Joplin district, Missouri, a preliminary report: Tech. Paper 41, Bureau of Mines, 1913, p. 36. Richards, Robert H., Ore dressing, vol. 3, 1909, p. 1549. If a mixture of good and poor conductors, in a neutral state, be dropped upon a highly charged conducting surface, the good conductors immediately receive a charge similar to that of the surface and are repelled, while the poor conductors are much more loath to receive the charge and therefore not so readily repelled. If, however, a material charged to a high potential of the opposite sign be fed to the above-mentioned surface, the good conductors, as before, assume immediately the condition of the charged surface and are repelled, while the very poor conductors, carrying a charge opposite to that which the surface carries, by the above law, tend to cling to that surface, thus making a sharper division of the separation. Theoretically the separation of two minerals does not require that one of them be a very good conductor and the other a very poor conductor, but merely that there be a difference in the degree to which they will conduct electricity. Commercially, of course, there are limits to this. At Midvale, Utah, near Salt Lake City, at the plant of the United States Smelting, Refining & Mining Co., an electrostatic concentrating plant has been in operation since 1909, that is said to save 5,000000 to 7,000,000 pounds of zinc annually which before had gone to waste. Zinc-iron middlings, partly furnished by the company's concentrator, partly from custom ore from the district, are passed over a Huff separator. MacGregor a states that the "feed" to the mill assays approximately 0.04 ounce gold, 2.5 ounces silver, 2.5 per cent lead, 1 per cent copper, 23 per cent iron, 28 per cent zinc; the separated zinc product assays 48 to 50 per cent zinc with 3 to 5 per cent iron, and the iron product 10 per cent zinc. Most of the gold, silver, copper, and lead is found in the iron product, and this product is smelted in the company's lead furnaces. The iron product contained about 10 per cent zinc. The assays of other products, as given by MacGregor, show the same thing; that is, in the "iron product" or the "copper product"-the product that goes to the blast furnace and from which the greater part of the zinc has been separated-there is still anywhere from 8 to 12 per cent zinc, and the zinc in this product is not recovered. FLOTATION PROCESSES. In order to concentrate a zinc ore by flotation it is generally necessary to heat the pulp and to use acid. If the ore contains both lead and zinc the minerals carrying these metals are floated, and the lead-bearing minerals must be separated from the zinc-bearing minerals. In order to effect such a separation, Horwood, of Australia, devised a selective or preferential process. The following statement in regard to this process is taken from Metallurgical and Chemical Engineering. Selective flotation by Horwood's process.-The Broken Hill correspondent of the Australian Mining Standard, January 15, 1914, states that the successful establishment of a MacGregor, F. S., Progress in electrostatic ore dressing: Trans. Am. Electrochem. Soc., vol. 24, 1913, p. 352. News item, Selective flotation by Horwood's process: Met. and Chem. Eng., vol. 12, May, 1914, p. 350. the Horwood process of selective flotation at the works of the Zinc Corporation was one of the metallurgical achievements of 1913. The principle of this process is the roasting of mixed lead-zinc concentrates obtained by flotation, whereby the lead particles are slightly sulphatized while the zinc is unaffected. On floating the roasted product the zinc alone floats and the lead remains submerged. A plant having a capacity of 500 tons weekly has been in operation for over six months, treating current and accumulated slime concentrates. The slimes are first dewatered and washed to remove soluble salts, then lightly roasted in an Edwards furnace, and finally treated again by flotation. Typical actual results from this plant are as follows: The grade of the lead is, of course, dependent on the amount of gangue in the feed. Tests made on Tasmanian ores show that 85 to 90 per cent of the zinc can be recovered as a concentrate assaying 57 to 58 per cent zinc, while the lead will remain with the iron content of the ore. From 75 to 80 per cent of the silver content of the ore and about 90 per cent of the gold will accompany the lead residue. Other differential flotation processes that have been more recently developed are mentioned on a later page of this bulletin. ZINC RECOVERY BY DIFFERENT CONCENTRATION PROCESSES. Table 2 following shows the average recovery of zinc by various standard concentration processes. Most of the present processes rarely give a clean separation of lead and zinc; the product that goes to the zinc smelter contains considerable lead, and vice versa, the zinc contained in the lead product or the lead in the zinc product being lost. Only the zinc lost in smelting lead concentrates in a blast furnace is considered; the flotation of zinc ores is described in a later chapter. 86198-19-Bull, 168 TABLE 2.-Recovery of zinc by various concentration processes." a Lyon, D. A., and Arentz, S. 8., Losses of zinc in mining, milling, and smelting: Trans. Am. Inst. Min. Eng., vol. 49, 1915, p. 799. Engineering and Mining Journal, Magnetic separation of zinc blende at Denver, Colo., vol. 74, Aug. 16, 1902, p. 217. p. 352. MacGregor, F. S., Progress in electrostatic ore dressing: Trans. Am. Electrochem. Soc., vol. 24, 1913, d Hoover, T. J., Concentrating ores by flotation, London, 1912, pp. 91, 97, 105. LOSS OF ZINC IN LEAD SMELTING. In smelting a zincky lead concentrate in a blast furnace the metallurgist seeks to get rid of the zinc either by fluxing it with the slag or by volatilizing it as fume, and this zinc is lost. Not only does the miner receive no return for any zinc in the lead concentrates he sends to the smelter, but he is penalized if the proportion of zinc is more than 8 to 10 per cent, because the presence of a larger percentage of zinc causes serious irregularities in the working of the lead furnace and decidedly increases the cost of smelting the ore, more coke being required per ton of ore smelted and the lead-producing capacity of the furnace being lowered. As Anderson a has pointed out, it is impossible to express in writing the difficulties met in attempting to smelt zincky ores. To illustrate the amount of zinc lost in this country in smelting zincky lead ores, the following example will serve: A concentrate containing, say, 28 per cent of zinc is separated into two products, one containing, say, 50 per cent zinc and the other containing most of the gold, silver, copper, and lead, and about 10 per cent zinc. Then of the original 50 tons of concentrate a Anderson, L. D., Some methods of recovering zinc from its ores: Utah Soc. Eng., Salt Lake City, Utah, May, 1916. Lyon, D. A., and Arentz, S. S., Losses of zinc in mining, milling, and smelting: Trans. Am. Inst. Min. Eng., vol. 49, 1915, p. 798. |