The STORY of ZINC. I +
H. R. HANLEY*
School of Mines and Metallurgy, University of Missouri, Rolla, Missouri
The contents of this paper embrace the historical account of the metal, the part it played i n the alloyed form before it was known as a separate entity, the fundamental concept of four processes used for its production together with the practice of the same, and the uses of the metal. The paper will run serially in the JOURNAL. The first instalment relates the history of the metal, the thoretical and practical aspects of the retort process, and the influence of modern milling methods on zinc metallurgy.
copper, bronze, and brass without evidence of any differentiation. In the fourth century B.C. Aristotle (3) describes an Asiatic race, the Mossynceci, living in the neighborhood of Calchis near the Black Sea. They produced a white or brilliant copper by melting copper with an earth found there. The Romans as early as 200 B.C. were well acquainted with brass, but not so the Greeks (4). The former knew that certain "earths" had the property of coloring copper a beautiful golden yellow. This alloy corresponds to what would now beclassified as a low-zinc brass. EARLY HISTORY In more recent times this alloy has been made experiETALS came into use very slowly with the mentally by Peter LeNeve Foster (3), who reduced auriadvancement of primitive man and his progress chalcite, 2(ZnCn)COa.3(ZnCu)(OH)%, a basic carbonate is definitely recorded along with his ability to dis- of zinc and copper, with charcoal in an assay furnace. cover and exploit them. There were six metals in use The only method of production of this alloy in the prior to the Christian Era (I): gold, silver, copper, early part of the Christian Era was by the cementation iron, lead, and tin. Mercury was known but was not process, whereby metallic copper is heated with an classified as a metal because of its fluidity a t room tem- oxidized zinc ore (calamine) and charcoal. There is perature. It is apparent that the definition of a metal an authentic record of this. The zinc reduced in this was obscure a t this time but probably no more so than manner alloys directly with the copper. Paracelsus, it is today, except in terms of itself. who died in 1541, wrote of zinc as a metal, using the There is some uncertainty as to any definite date word einken to designate it. prior to A.D. 1509, when recognition was given to zinc The first recognition of metallic zinc in Europe was by as a separate metal. P. Neogi (2) has stated that zinc Erasmus Ebener of Nurnberg who, in 1509, separated was produced as a separate metal in India as early as i t from the zinc slag of silver-smelting works on the the seventh century. H. T. Chang (2) states that zinc Rammelsberg (5). He made brass direct from the was similarly produced in China as early as the eleventh globules of zinc so recovered. Lohneys in 1617 decentury. Mr. Chang cites from the literature of that scribes the finding of a metal called zinc in the cracks period that coins were then made containing eight parts of furnace walls (6). Agricola made a similar discovery of copper, four parts of black tin (lead), and two parts of a few years later. Percy records the condensation of zinc; their actual analysis reveals that these coins zinc on the walls of a blast furnace smelting zincky iron contained 55% Cu, 26% Pb, and 13% Zn. A close ore in South Wales (3). Glauber-made the discovery analytical check of facts as stated in the ancient litera- of the mineral causing the metallic zinc accretion and ture is offered by Mr. Chang as evidence that the Chi- announced that calamine was an ore of zinc (6). nese not only produced zinc as a separate metal but The properties of a metal are always significant in made their coin metal by mixing copper, lead, and shaping the course of the metallurgical procedure of zinc. winning it in metallic form from its ores. The property Zinc was not known in Europe as a separate metal of zinc, which thus dictates its pyrornetallurgy, is its for over 1500 years after the beginning pf the Chris- high volatility a t temperatures below the commercial tian Era; therefore as a separate entity it played no reduction temperatures of its compounds. Consepart in the affairs of ancient man in Europe. Hence it quently the primary product obtained is not liquid may be classed as a metal of relatively recent discovery. metal, but a vapor from which the metal must be conThis is not true, however, with respect to zinc as a com- densed under suitable thermal and other physical conponent of the alloy, brass, which consists of varying ditions. In view of this fact, it is not strange that percentages of zinc and copper and, frequently, other ancient man, without chemical and physical knowledge metals as impurities. of zincand its compounds, was unacquainted with this It is presumed that copper was the first metal worked metal in its metallic form. by man and the time of its first being alloyed with other The first production of zinc on an industrial scale metals is placed before the beginning of history. The was made in 1740 a t Bristol, England, by John Chamearliest - classical writmgs make frequent references to pion. He availed himself of the glass furnaces in use * Mining and metallurgical engineer, professor of metallurgy. a t this time and developed the so-called English method 61DO
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of distillation in large pots. This procedure he kept secret for a long time. The next pioneer in the field was John Christian Ruberg, who in 1800 established a zinc reduction plant a t Wessola, Poland. He found defects in Champion's process and sought to improve the technic. This he did by evolving the system of horizontal muffles of large capacity, destined to form in later years the basis of the great Siesian zinc industry. Contemporaneously with Ruberg's work, Dallinger developed zinc distillation in Corinthia. One of the most outstanding pioneers in the field was Abb6 Dony of Li6ge, who, after years of development, discovered independently the pyrometallurgical process for reducing zinc ores. Dofiy, in 1807, established works a t Li6ge which were destined to mark a great epoch in this industry through the origination of the Belgian method of extraction in use in a modified form a t the present time. It was difficult to market the new zinc in the form of brass because of the effortsof the well-established calamine-brass makers who did not relish the encroachment on their business. They contended that the new product was inferior to cementation brass. These objections were well founded as the first brass made by melting copper and zinc was of poor quality. Abbe Dony was ruined financially and died a broken-hearted man. His plant continued, however, under various successors. One group of these founded the great company known as the Soci6t6 de la Vielle Montagne. a metallurgical enterprise with hardly an equal in the world and which has retained its name to the present time. The history of a metal embracing its influence on civilization leads far afield in the story of its production. Dr. T. A. Rickard has recently written a much needed work entitled "Man and Metals." in which he closely links civilization with the use of metals. A new book by T. T. Read, "Our Mineral Civilization," is also of interest on the subject of the early use of metals. In the United States (7) the first zinc was produced by John Hiltz in 1835-6. He used the mineral zincite as the source and carried on the work in the arsenal a t Washington, D. C. The following chronology records the early efforts which followed those of John Hiltz: 1850, New Jersey Zinc Co., Newark, N. J. 1855, A. M. Anderson, Potosi, Mo. 1856, Lehigh Zinc & Iron Co., Friedensville, Pa. ? 1860, J. Wharton, South Bethlehem, Pa. 1860, F. W. Matthiessen and E. C. Hegeler, La Salle, Ind. 1870 to 1880, Chicago Zinc & Mining Co., Weir City, Kan., and Pittsburg, Ran. The Lehigh Valley companies were really the beginning of the American zinc industry but Matthiessen and Hegeler operated the first successfulzinc reduction plant. GENERAL PRINCIPLES OF ORE TREATMENT
In the course of the industry, metallic zinc is or has been produced by the following processes:
1. The retort process. 2. The electrolytic process. 3. The electrothermic dry-distillation process. 4. The electric smelting process. The production of metals from ores is concerned with one or more of the three types of reactions:
I. Carbon Reduction RO+C=R+CO 2RO C = 2RO COa CO = R COB RO R O + C & = R f CO +2Hz
11. Sulfide Reduction
RO RS = 2R C SO* 3R0 RS = 2R R1O h S = 6R SO* 2hO
++ + SO,
111. Precipitation RSf M = R + M S
R and M denote any metals participating in the reaction. The production of zinc is limited to the first type. Inasmuch as the reduction is applied to the oxide of zinc by carbon and carbon monoxide, the first step in any process for the production of zinc is that designed to produce the oxide. There are only two important types of zinc ores: the sulfide type and the oxidized type. The sulfide type embraces pure blende (ZnS), marmatites (5ZnS.FeS), and complex sulfide mixtures such as: ZnS, FeS, PbS, 5ZnS,FeS, etc. This type of ore or concentrate must be as completely desulfurized as is economically feasible to permit the free play of carbon and gas reduction to take place in a subsequent step in the process. This desulfurization is termed roasting o r sintering. The oxidized type, embracing carbonates and silicates, is treated for the production of oxides by a process known as calcination-similar in principle and practice to the burning of limestone to lime; that is, the ore must be heated in the presence of air under proper conditions for elimination of coplliined carhon dioxide and water. This type of ore is usually termed calamine, regardless of the acid radical components. The elimination of the combined substances in calamine, although not absolutely necessary for the subsequent carbon and gas reduction, is usually carried,out. The sulfide type of ore predominates in the United States and Europe, although calamine was more plentiful in Europe a generation ago. The retort process accounts for approximately 70% of the zinc production of the world. The following description applies to the treatment of the sulfide type of ores or concentrates by this process, as shown in Figure 1, which is a generalized flow-sheet. Blende or zinc sulfide concentrate, containing 60% Zn, is introduced into a furnace where the raw material is deprived of its sulfur by a process of roasting. The desulfurized concentrates must be in a granular condition when they are introduced into the retort for reduction and distillation, otherwise the flow of gas and zinc vapor becomes
blocked. Modem millmg methods produce a very reduction operation. Approximately 16 to 18 hours finely divided concentrate which must not only he de- are required for the extraction of the zinc in the charge. sulfurized but also made granular by special pro- After the condensers have been tapped for their cedure. This is accomplished by partial desulfurization content of liquid zinc, they are taken down, the resiin a multiple-hearth roasting furnace followed by dues are removed from the retort, a fresh charge is introsintering which completes the desulfurization. The duced, condensers are replaced, and the procedure is sinter cake is then properly ground to produce the repeated. The flow-sheet shows sulfur dioxide flowing from the roasting COARSE FLOTATION (100 mesh) CONCENTRATE CONCENTRATE furnaces to the sulfuric acid dant. Retort plants situated near indusI I trial centers have a market for On TO HpSOl PLANT WASTE GAS sulfuric acid and frequently the sale of this by-product is the MULTIPLE chief source of revenue to the zinc C O A a 7 HEARTH company. There are laws preRfkZT WATER-*, .l venting the releasing of quantities of obnoxious gases into the atmosphere in populous areas, consequently the sulfur dioxide coming from the roaster must be converted into acid, regardless of revenue. Similar plants whichare not situated near a market for DWIGHTLLOYD their acid, may send the roaster SINTERING MACH. gas to the atmosphere through a high stack. This serves to effect a great dilution of gases with such SINTER CAKE consequent diffusion and dilution CRUSHED & GROUND as to render them quite harmless. I roast in^ furnaces in such plants are of the reverberatory type, in which the fuel and ore gases are not separated. They may he of the most economic design, as for example, a sinple-hearth straiphtline furnace. ?his flow-sheetdesaiption is intended merely to indicate the progress of materials through a generalized hypothetical retort zinc plant. No reference has been made to the problems encountered in each division RETORT RESIDUE CONDENSED ZINC of the plant. nor to the ~roblems of major importance which the BROKEN CONDENSERS ARE CRUSHED, industry has experienced in the GROUND AND TABLED FOR ZINC DRILLS. past several Aside from the casual improvements in methods common to indesired granular condition. The coarse concentrates dustrial plants in general, there appear, a t remote may be commercially desulfurized in the roasting fur- intervals, revolutionary alterations in practice to cope nace. From the storage bin the fully conditioned con- with changed conditions. Such changes may be classicentrate is delivered to a mixer, where it is incorporated fied in the following manner: -first, changes in the with 50y0 to 60y0 of reduction coal. A small amount of nature of the raw material, demanding fundamental alwater to provide greater porosity is introduced and in terations in methods and equipment; second, changes some cases salt is used in small amounts. The mixture in policy, due to changes in the economic situation, is then introduced into the retorts which are operated which may result in the conservation and beneficiation a t a temperature of approximately 1200' to 1300°C. of more of the potential by-products; and third, deand the condensers are then luted in place with mois- velopment of machinery which may render older equiptened slack coal and clay. This is the beginning of the ment obsolete.
The grade of the ore may fall below the economic (plan), holding a prepared charge 5" to 8" in depth. limit for commercial treatment by the old methods in These molds, called pallets, fit closely together and form use and compensation is sought by a more complete ex- an endless conveyor with the continuity broken a t one traction of the minerals in the ore, together with in- point in the circuit. creased tonnage. The possibility for increased extracEach pallet is provided with four wheels which engage with tion of the minerals lies in the finer of the or guides a t all parts of the circuit except where the pallet ores thus liberating them from their locked association Facks 1s passing over the suction box. Then the pallet slides along with with the waste particles. It is necessary its planed base directly over the smooth top of the suction box. that the ore be mound to aass through a 200-mesh thus makine- a tieht - -ioint. A com~letecycle of a ~ e r a t i o uis as gravity concentration follows: a pallet, being pushed onward tangent& from the screen. The 016 method cannoteconomically be applied to this extremely fine top of the sprocket wheels, passes under the feed hopper where it takes its load in the form of a continuous even Layer of charge, of ore, but by the process of selective flotation the heavy appmfimately four inch- in thickness; i t then passes under the minerals may be floated away from the lighter waste igniter where the top surface is kindled and a t the same time material. Inasmuch as the retort vlant demands a comes under the iduence of the downwadmoving current of granular material for reduction and distillation, this air, induced hy the suction draft; this carries the sintering action concentrate must be given special =ondition- progressively downward until it reaches the grates; the roastsintering action is now complete, the cake is discharged by the ing during desulfurization, thus destroying its amor- palletwhich drops it into the discharge-guides; the pallet crowds phous* character. its way hack to the sprocket wheels, is slowly raised to the upper The increased extraction of zinc minerals by means of tracks, and begins a new cycle (8). the flotation process greatly increased the ratio of flotation to granular concentrates.t Hence it beThe Rigg method of desulfurization came necessary for zinc plants to rebuild comand sinteringconsists pletely their desulfurization departmentsa very in roasting the concostly procedure. Many of the old straight-line centrate so that it 9-k furnaces that operated a t high temperature (850' to 900°C.) have been scrapped to some extent contains a p p r o x i mately 8% to 10% or permitted to remain idle. Modern sulfide sulfur. This is mixed with the equipment emhracing multiple-hearth p r o p e r amount of furnaces and the water and the dampDwight-Lloydsinterened charge is delivered to the moving mach'me has been ing grates where i t substituted therefor. issubjected to a sucMultiple-hearth fur~ c ~ ~ r o Book ~ CO., i ~ Inc. l tion of from 12" to naces can also withstand high temperaFIGURE2.-DWIGHT-LLOYD SINTERINOMACHINE 15'' of water. It is n o w i g n i t e d by tures when adequate The incandescent charge means of an inverted muffle. provision is made for heat-resisting alloys for the rabis under the influence of the vacuum for approximately brig mechanism. , 25 minutes. DESULFURIZATION AND SINTERING In the Baelin method the sulfide snlf& is lowered to The zinc industry found the sintering process already a greater degree by roasting so that the calcine contains developed for lead ores, but to apply i t to zinc con- from 1% to 5% sulfide sulfur. It is mixed with sufficentrates, considerable study and experimentation was cient coal to give a combined sulfur-plus-coal content necessary, particularly in the partial roasting step of 8%; it is then dampened and sintered. The charge which precedes sintering, and in the design 6f ignition delivered to the sintering machine must poss'ess a high burners. Lead ores or lead concentrates are v q w i l y permeability so as to permit easy passage of air and sintered and the sinter contains from 1% to 4% gases. Permeability is imparted by interstitial and sulfur which is permissible and sometimes desirable. grain porosity. The grain porosity may be produced Zinc concentrates** are more diflicult to sinter and the by developing zinc sulfate during the partial roasting desulfurization must be as complete as is economically operation. The presence of zinc sulfate per se in the possible. grain is of little direct significance but denotes a The Dwight-Lloyd sintering machine (see Figure 2) satisfactory porosity. When zinc sulfate is present, its consists of slowly moving molds having perforated decomposition is greatly facilitated by the following grate-bottoms. '?hey are-approximately 24" X 42'' reactions:
' T h e word "amorphous" is used in a liberal sense.
ratio of flotation to granular concentrates: 112.5. ** T ~ Cterm "zinc concentrates" is a general one. The exact phrase would specify the mineral such as "blende concentrate, "mannatite concentrate," etc.
(1) ZuS (2)
++ ZnSO. = 2Zn + 2502 3ZnS01 = 4Zn0 + 4SOn
~h~ zinc vapor in equation(1)is probably simultaneously condensed and oxidized resulting in the formation
of needle-like aystals of zinc oxide. Sulfate usually forms after two-thuds of the sulfur has been oxidized. In the case of nearly complete desulfurization in the roasting furnace, the last portion of the sulfide sulfur to be oxidized is changed direct to sulfate, providing the temperature is not above that of sulfate decomposition. The second factor of permeability, that of interstitial space, is developed by the addition and mixing of the proper amount of water. There is a critical ratio between the weight of water and the weight of dry solids which gives a maximum specific volume of the untamped mixture. This maximum specific volume is of very great importance. Water, when in proper ratio to the solids, provides maximum distances between solid particles and when the water is removed by evaporation these distances provide interstitial cavities having similar dimensions. The optimum ratio varies greatly, depending on the physical nature of the solid material. Faxtially desulfurized concentrate having a high zinc sulfate content can absorb a large amount of water and still be dry to the touch. The incorporation of water into a sulfated calcine develops heat and causes a rapid evaporation of moisture. This feature renders necessary the immediate sintering of a properly dampened charge, otherwise additional water becomes necessary. The variable water requirement is related to the water of aystalliiation of the salt. The formation of zinc sulfate may be explained as follows. The SOB generated in the roasting is partly oxidized to SO2 by the catalytic effect of the hot charge and the hot furnace walls. This SO2 combines with ZnO forming ZnS04. In addition to this there is some zinc sulfate formed directly from the oxidation of the blende, thus: ZnS
+ 2 0 2 = ZnSO,.
This reaction is very pronounced during the oxidation of the last sulfide particles. A varying amount of zinc sulfate usually forms in roasting. Because a high temperature (900°C.) is necessary for its decomposition, all furnaces prior to the introduction of sintering were built to withstand such heat and were usually of the single-hearth, straight-line type. Hofman and Wanjukow, in experimenting on the decomposition of the sulfate, obtained the following data:
Zinc sulfate may also be decomposed without excessive beat by introducing raw sulfide concentrate into the last hearth of the furnace. The decomposition is represented by the equation: ZnS
+ ZnSO. = 2Zn + 2SOn.
The zinc vapor in this case is immediately oxidized and collected either inside the furnace or in the baghouse. The necessity for the complete decomposition of zinc sulfate into oxide in the prepared concentrate prior to its introduction into the retort is due to its reducibility by carbon to sulfide. Inasmuch as no metallic zinc results from this reaction, every unit of zinc as sulfate is lost to the process. Lime, to a slight extent, is a frequent impurity in zinc concentrates. This is changed to calcium sulfate in the hearth roaster where the prevailing temperature is insufficientto cause its decomposition. Because lime combines with sulfur to form sulfate, it is objectionable in a roasting system which uses the SO2 gas for acid manufacturing. % cso *rrscnt in Ihc ore
Calcium sulfate is completely decomposed in the sintering process. The temperature of the hottest part of the sintering charge seldom reaches 1050' to llOO°C., this being 100° to 150' below its decomposition point by heat alone. It is probable that this salt is chemically decomposed in a manner similar to the decomposition of zinc sulfate, or according to the following reaction: CaSO'
+ 4C = CaS.. +.4CO.
The resulting incandescent calcium sulfide, under the iduence of excess air, is completely desulfurized. In the Rigg method, sulfates probably play a small part in the general scheme.
RICKARD, T . A,, op. cil., p. 152. Re Metallica," Translated by HON. A m A o n r c o ~ ~"De , Mns. H. C. HOOVER, published by The Mining Magazine of London. 1912. HOPMAN,H. 0..op. cil., p. 4. HOFMAN,H. O., "Metallurgy of lead," McGraw-Hill Book Co., Inc., New York City, 1918, p. 192.. , HOFMANAND W A N J ~ O W"Dewrnpos~t!on , of metallic sulphates," Trans. Am. Inst. Mining Mct. Engrs., 43, 563 (1912). . (Parl II will appear i n lhc November isszre.)
T. A,, "Man and metals," Wittlesey House, Book Co.. Inc.. New York City. 1932. McGraw-Hill - vol. I, p. 6. READ,T. T.."R6Ie of mining in the develop~entof civilization," Eng. Mill. J., 133, 633 (1932). MOULDEN. J. C., "Zinc, its production and industrial applications," J. Royd Soc. qf Arts, 64, 495 (May 26, 1916). HOFMAN, H. 0.. "Metallurgy of zinc and cadmium," McGraw-Hill Bwk Co., Inc., New York City, 1922, p. 2.
m The nttentiorl of c o r r c ~ ~ o n d c n tisr directed to the chande of address of the editorid office, which is now located a t Keut rhe business and puhliration ofires remain a t the plixnt of the Chemical Lsborntory. The Univerntv olChmcn o .\lack Printing at Enston. Pmns,yl\,ania.