The STORY of ZINC. IV*


These bri- quettes are then subjected to reduction by heat produced by. THE ELECTROTHERMIC DRY-DISTILLATION PROCESS. T HE ELECTROTHERMIC ...
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The STORY of ZINC.

IV*

H. R. HANLEY t School of Mines and Metallurgy, University of Missouri, Rolln, Missouri

This inslalment, which is the fourth and concluding one in "The Story of Zinc," embraces the electrothermic distillation and also the electric smelting method of production. I n the former process, the roasted ore and reducing carbon are formed into briquettes. These briquettes are then subjected to reduction by heat produced by

the $ussage of an electric current through them. Largescale condensers are necessary. The latter process (electric smelting) encom$usses many operations; namely, smelting to form metal matte and slag, distillation of the zinc as an impure metallic powder, and the beneficiation of the same into commercial zinc.

(3) Practically complete extraction of the zinc from the ore. HE ELECTROTHERMIC dry-distillation proc(4) High recovery of zinc--most of it as liquid metal. ess was developed by Dr. C. H. Fulton and has ( 5 ) Low power-consumption, less than the electrobeen fully described in the technical press (38). thermic smelting and less than the electrolytic The essential features of the process are published in the process. Engineering & Mining Journal. The process includes (6) Applicable to all types of ores. the briquetting of a mixture of roasted concentrate, (7) No regular consumption of fire clay; it can be coal, and hard pitch. The .briquettes are then subbuilt in small units in inaccessible places where jected to primary reduction in a separate furnace, electric power is available. heated by means of hot residue-briquettes. The Inasmuch as the primary reduction is performed in residue-briquettes contain practically nozinc but retain their original form. After the primary reduction, the a separate furnace and because the charge is under prachot briquettes are assembled between the terminal tically isothermal conditions, there should be little soelectrodes of an electric circuit and constitute a resistor called blue powder, chemical or physical. This process which may be heated to any desirable temperature awaits large-scale mechanization. like a lamp filament in an electric light bulb. An elecTHE ELECTRIC SMELTINGPROCESS tric distillation furnace was built in East St. Louis, This process consists of heati3g roasted complex zinc Illinois, and treated a briquetted charge of approximately 3200 lb., consisting of twelve columns of three ores by means of an electric current, passing through briquettes each. The dimensions of these briquettes graphite electrodes immersed in the ore. The copper and some of the lead are tapped as molten products and were 91/n" in diameter and 21" high. the impurities are carried away in a slag that entrains The energy is applied directly to the charge and is not trans- but little zinc. The zinc and the retdainder of the lead mitted to it through walls of refractory ma'terial as is done in the retort process. This feature permits an approwh to the theo- are distilled and enter a condenser that receives a retical energy consumption. The energy consumption at the mixture of these metals as metallic powder. The conEast St. Louis experimental plant was approximateiy 1500 densate is introduced into an electrically heated rekw.-hr./ton of concentrate (39). , volving cylinder from which air is excluded. The rotary motion induces friction between the surfaces of The process is applicable to the high-grade ores and the myriads of oxide-coated metallic particles of zinc concentrates now used in the retort process and to comand lead with the result that a mdten alloy is produced. plex ores containing iron, lead, silver, and copper. The separation of the lead from the zinc is accomplished The costs of construction and operation of a plant by liquation. This is the process of slowly heating based on any process under present conditions serves an alloy placed on a sloping hearth to a temperature no useful purpose but a statement of characteristics slightly above the melting point of the most easily inherent in a particular process conveys information of fusible metal. The liquid metal then gravitates away interest. The characteristics of this process may be from the metal having the higher melting point. The summarized as follows: zinc produced by this process always has a lead content Large-scale units mechanically operated as in the comparable to that of the zinc produced from leady (1) zinc concentrates by the retort process. metallurgy of other common metals. This process has been operated only in Sweden where (2) Low labor cost, mainly unskilled. hydroelectric power is lower in cost than elsewhere. * Part I appeared in the October, 1933, issue; Part I1 in No- It is not being operated on a commercial scale a t the vember, 1933; Part I11 in January, 1934. t Mining and metanurg~calengineer, professor of metallurgy. present time. 111 THE ELECTROTHERMIC

T

DRY-DISTILLATION

PROCESS

GRADES OF ZINC

There are four commercial grades of zinc, the analyses of which are shown in Table 6.

.. . ..... .

0.07 0.03 0.07 0.10 .... 0:50 0.03 0.20 0.50 . .. 0.50 0.03 0.60 l.W .. . . 0.08 1.60 . .. .... and r Am. Sae. T u t . Mat., Pro'., Vol. XVI, pp. 583-9; Vol. XVII, pp. 180-1 (1917). and d Am. Soc. T u t . Mat., Proc., Vol XI, PP. 147-9. nigh grade' Intermediateb Br- Special' Prime Wuternd

..

.

..

High-grade zinc is produced from ores free from metals which are volatilized a t the reduction temperature of zinc, or if volatilized, are not condensed a t the condensing temperature of zinc. These ores are free from lead and cadmium. All zinc produced electrolytically is classified as high-grade zinc, some brands containing only 0.01% of impurity. USES OR ZINC

There are four major uses of zinc: galvanizing which consumes 45% of the total production; brass, 25%; rolled zinc, 14%; and die casting, '7%. The balance is represented by zinc dust, pigment, wire cloth, etc. The uses of zinc in large quantities are not so diversified as the major uses of copper and lead. The uses of copper and lead are such that certain percentages are returned to the market as secondary or scrap metal. When conditioned or refined, this competes with virgin metal. This is not true for the major uses of zinc and with the exception of rolled zinc, die castings and brasses, the zinc metal going into industrial uses does not reappear on the market as secondary metal. Galvanized steel sheets, now more properly termed "zinc-coated steel sheets," formerly were subjected to uncertainties of trade, partly because of questionable coating practice and partly becausP'of the use of paint as a substitute therefor. However: due to a vigorous and meritorious campaign inaugurated by the American Zinc Institute, these uncertainties have been relieved. The inimitable value of zinc-coated sheets when the coating amounts to 1v4to 2 oz. per sq. ft. of surface has been demonstrated. A life of from twenty to thirty yeas is assured by the use of this weight of coating. Consequently a return of the buying power of the great consumer-the farmer-will materially stimulate the consumption of zinc. The process of galvanizing is adequately described in the bulletins of the American Zinc Institute and also in books on the subject by Bablik (40) and Imhoff (41). The essential preliminary step in either hot-dip or electro-galvanizing is that of preparation of the steel surface. This preparation is accomplished by a process of pickling; namely, the dissolving of the surface oxide coating in acids and the prevention of tarnish prior to the application of the coating. Pure zinc is not suitable

for hot-dip galvanizing because of its high viscosity a t the temperature employed. Lead is a common impurity in zinc used for this purpose. Aluminum has been used to the extent of 0.63% to lower the oxide content of the surface and to make thinner coatings. But since thin coatings are no longer desired in galvanizing, the the use of aluminum has declined. The galvanized surface of steel always shows a covering of beautiful spangles. These are dependent upon the following features: (1) composition of the steel base, (2) rate of crystallization, (3) method of cooling. They are further influenced by two circumstances, namely, the chemical composition of the outer coating and the length of time maintained a t the temperature of the melting point. The addition of tin occasions very large spangles due to the maintenance of a larger crystallization interval. Zinc is also applied as a coating to irregularly shaped pieces of steel by a process of cementation, known as Sherardizing. The articles are packed in zinc dust in a drum, heated electrically to 400┬░C., and rotated for several hours. They take on a dull light-gray coat of zinc which increases in thickness with temperature and time of treatment. The Schoop Metallizing Process consists in spraying atomized zinc against steel with great force. The zinc in the form of a ribbon is conditioned by an oxygenhydrogen blow-pipe flame, having an excess of hydrogen. The theory of operation is that the heat set free by the impact of the metal upon the steel surface momentarily vaporizes the metal which then condenses and solidifies. The most important furnace method of producing zinc oxide is known as t2le'WetheriU process. This consists of heating a mixture of reducing coal and roasted ore or natural oxide ore on a perforated grate. An air blast is forced up through the charge. The zinc oxide in the calcine is reduced to vapor which burns to a pure oxide in a, combustion chamber. Thence the gas stream is delivered to a bag house where it is filtered, the zinc oxide collecting on the inside of the bags. This product alone, or mixed with lead pigment, is used extensively in paints. Zinc oxide is also used extensively in the manufacture of automobile tires which contain from 25% to 55% of it. A large increase in the use of zinc bas been occasioned by the die-casting industry. The manufacture by this method of mechanical devices composed of zinc alloys greatly diversifies its uses. Die-cast grease guns, carburetor parts, ornamental hardware, supports, and light fixtures are produced with surfaces so smooth and accurate that no machining is necessary. Porosity has been an obstacle to the extended use of zinc but this disadvantage has been overcome by alloying with suitable metals. Such alloys containing copper, aluminum, and magnesium are particularly well adapted for die casting. Hence in a period of six years, the zinc tonnage used in this way has increased from 2.1% to approximately 7% (42).

In this story of zinc it has been stated that the metal was h t used in the alloyed form before it was known as a separate entity. Today we have the advantage of knowing zinc not only in the commercial form, in which it contains the customary limit of impurities, but also as an element of practically 100% purity. Our ever-increasing knowledge of metallography is a great potential factor and a stimulus in the development of new alloysalloys possessing new properties of great usefulness. In one small division of its uses, namely, die castings, we observe a threefold increase in six years. It is probable that the full potentialities of this metal have not yet been realized and that we are approaching an alloy age in zinc which may further widen the diversification of its use. As this becomes

a reality, the 55% quota not used for galvanizing will be more in demand than a t the present time. The properties of zinc not only shape its metallurgy, but are also associated with the factors that influence the selection of this metal for use. LITERATURE CITED

C. H. FULTON, ''Apropo~edplant for the electrothemic drydistillation of zinc ores," Eng. ildining 3.. 114, 13 (1922). W 0 .. -HOPMAN. - - - ~ - "Metallurev ~ ~ - , of zinc and cadmium." 1st ed.. McGraw-Hill Book co.:inc.. New York City. 1922, p. 14: HEINZ BABLIK, "Galv&izin&," D. Van Nostrand Co., New York City, 1926. WALLACE G. IMHOPP, "Pickling of iron and steel," Penton Publishing Co., Cleveland, Ohio, 1929. Massachusetts Institute of Technology, Tech. Re& (April, 1933) (This instalmcnt concludes the series.)