Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
ZINC
Online EncyclopediaEncyclopedia Home :: YAK-ZYM
del.icio.us it!
|
ZINC , a metallic chemical See also:element; its See also:symbol is Zn, and atomic See also:weight 65.37 (0=16). Zinc as a component of See also:brass (XaXKbs, opei-xaXeos) had currency in metallurgy See also:long before it became known as an individual See also:metal. See also:Aristotle refers to brass as the " metal of the Mosynoeci," 2 which is produced as a See also:bright and See also:light-coloured xaXK6s, not by addition of See also:tin, but by fusing up with an See also:earth. See also:Pliny explicitly speaks of a See also:mineral KaSµeia or cadmia as serving for the See also:conversion of See also:copper into aurichalcum, and says further that the See also:deposit (of zinc See also:oxide) formed in the brass furnaces could be used instead of the mineral. The same See also:process was used for centuries after Pliny, but its rationale was not understood. See also:Stahl, as See also:late as 1702, quoted the formation of brass as a See also:case of the See also:union of a metal with an earth into a metallic See also:compound; but he subsequently adopted the view propounded by Kunckel in 1677, that " cadmia " is a metallic calx, and that it dyes the copper yellow by giving its metal up to it. The word zinc (in the See also:form zinken) was first used by See also:Paracelsus, who regarded it as a See also:bastard or semi-metal; but the word was subsequently used for both the metal and its ores. Moreover, zinc and See also:bismuth were confused, and the word spiauler (the See also:modern spelter) was indiscriminately given to both these metals. In 1597 Libavius described a " See also:peculiar See also:kind of tin " which was prepared in See also:India, and of which a friend had given him a quantity. From his a.ccount it is quite clear that that metal was zinc, but he did not recognize it as the metal of See also:calamine. It is not known to whom the See also:discovery of isolated zinc is due; but we do know that the See also:art of zinc-smelting was practised in See also:England from about 1730. The first See also:continental zinc-See also:works were erected at See also:Liege in 1807. Occurrence. Zinc does not occur See also:free in nature, but in See also:combination it is widely diffused. The See also:chief ore is zinc See also:blende, or sphalerite (see BLENDE), which generally contains, in addition to zinc sulphide, small amounts of the sulphides of See also:iron, See also:silver and See also:cadmium. It may also be accompanied by See also:pyrites, See also:galena, arsenides and antimonides, See also:quartz, See also:calcite, See also:dolomite, &c. It is widely distributed, and is particularly abundant in See also:Germany (the Harz, See also:Silesia), Austro-See also:Hungary, See also:Belgium, the See also:United States and in England (See also:Cumberland, See also:Derbyshire, See also:Cornwall, See also:North See also:Wales). Second in importance is the carbonate, calamine (q.v.) or zinc spar, which at one See also:time was the See also:principal ore; it almost invariably contains the See also:carbonates of cadmium, iron, See also:manganese, See also:magnesium and See also:calcium, and may be contaminated with See also:clay, oxides of iron, galena and calcite; " See also: B. See also:Hoffmann, Zeit. ,f. See also:Berg. and Huttenwesen, vol. 41). 982 States. England, See also:France, Spain and See also:Austria have been fairly See also:constant producers. Germany produced 155,799 tons in 1900, and 198.208 in 1905; Belgium, 120,000 in 1900 and 143,165 in 1905; the t nited States, 111,000 in 1900 and 183,014 in 1905. The See also:world's supply was 445,438 tons in 1900, and 654,367 in 1905. METALLURGY The principles underlying the extraction of zinc may be sum. marized as: (1) the ore is first converted into zinc oxide; (2) the oxide is distilled with See also:carbon and the distillate of metallic zinc condensed. Oxide of zinc, like most heavy metallic oxides, is easily reduced to the metallic See also:state by See also:heating it to redness with See also:charcoal; pure red zinc ore may be treated directly; and the same might be done with pure calamine of any kind, because the carbon dioxide of the zinc carbonate goes off below redness and the See also:silica of zinc silicate only retards, but does not prevent, the reducing See also:action of the charcoal. Zinc blende, however, being zinc sulphide, is not directly reducible by charcoal; but it is easy to convert it into oxide by roasting: the See also:sulphur goes off as sulphur dioxide whilst the zinc remains in the (infusible) form of oxide, ZnO. In practice, however, we never have to See also:deal with pure zinc minerals, but with complex mixtures, which must first of all be subjected to See also:mechanical operations, to remove at least See also:part of the See also:gangue, and if possible also of the heavy metallic impurities (see ORE-DRESSING). As ores of zinc are usually shipped before smelting from widely separated places—See also:Sweden, Spain, See also:Algiers, See also:Italy, See also:Greece, See also:Australia and the Rocky Mountains region of North See also:America—it is important that they be separated from their mixtures at the mines. The difficulty in separating zinc blende from iron pyrites is well known, and probably the most elaborate ore-dressing works ever built have been designed with this end in view. The Wetherill See also:system of magnetic concentration has been remarkably successful in separating the minerals contained in the well-known deposit in See also:Sussex See also:county, N.J. Here very clean non-magnetic concentrate of willemite, which is an anhydrous zinc silicate and a very high-grade zinc ore, is separated from an intimate mixture of willemite, zincite and franklinites, with calcite and some manganese silicates. The magnetic concentrates contain enough zinc to be well adapted to the manufacture of zinc oxide. Magnetic concentration is also applied in the removal of an excess of iron from partially roasted blende. Neither mechanical nor magnetic concentration can effect much in the way of separation when, as in many complex ores, carbonates of iron, calcium and magnesium replace the isomorphous zinc carbonate, when some iron sulphide containing less sulphur than pyrites replaces zinc sulphide, and when See also:gold and silver are contained in the zinc ore itself. Hence only in exceptional circumstances is it possible to utilize a large class of widely distributed ores, carrying from 10 to 35 per cent. of zinc, in which the zinc alone, estimated at 2d. a See also:pound, is See also:worth from about 2 to £7 per ton of ore. The ores of the See also:Joplin See also:district, in the Ozark uplift in the See also:Mississippi Valley, are remarkable in that they are specially adapted to mechanical concentration. The material as See also:mined will probably not See also:average over Io per cent. of zinc, but the dressed zinc ore as sold ranges from 45 to 62 per cent. of zinc. This region now furnishes the bulk of the ore required by the smelters of See also:Illinois, Missouri and See also:Kansas. The ore, even if it is not blende, must be roasted or calcined in See also:order to remove all volatile components as completely as possible, because these, if allowed to remain, would carry away a large proportion of the zinc vapour during the See also:distillation. If the zinc is See also:present as blende, this operation offers considerable difficulties, because in the roasting process the zinc sulphide passes in the first instance into sulphate, which demands a high temperature for its conversion .into oxide. Another point to be considered in this connexion is that the masses of sulphur dioxide evolved, being destructive of See also:vegetable See also:life, are an intolerable See also:nuisance to the neighbourhood in which the operations take See also:place. For the desulphurization of zinc blonde where it is not intended to collect and See also:save the sulphur there are many mechanical kilns, .generally classified as straight-See also:line, See also:horse-See also:shoe, See also:turret and See also:shaft kilns; all of these may be made to do See also:good See also:work on moderately clean ores which do not melt at the temperature of desulphurization. But the problem of saving the sulphur is yearly becoming more important. In roasting a ton of See also:rich blende containing 60 per cent. of zinc enough sulphur is liberated to produce one ton of strong sulphuric See also:acid, and unless this is collected not only are poisonous gases discharged, but the See also:waste is considerable. When sulphuric or sulphurous acid is to be collected, it is important to keep the See also:fuel See also:gas from admixture with the sulphur gases, and kilns for this purpose require some modification. If hot See also:air is introduced into the See also:kiln, the additional See also:heat See also:developed by the oxidation of the zinc and the sulphur is sufficient to keep up a part of the reaction; but for the See also:complete See also:expulsion of the sulphur an externally-fired muffle through which the ore is passed is found to be essential. Distillation of the Oxide with Charcoal.—The distillation process in former times, especially in England, used to be carried out per descensum." The bottom of a crucible is perforated by a See also:pipe which projects into the crucible to about two-thirds of itsheight. The mixture of ore and charcoal is put into the crucible around the pipe, the crucible closed by a luted-on lid, and placed in a See also:furnace constructed so as to permit of the See also:lower end of the pipe projecting into the ash-See also:pit. The zinc vapour produced descends through the pipe and condenses into liquid zinc, which is collected in a ladle held under the outlet end of the pipe. For manufacturing purposes a furnace similar to that used for the making of See also:glass was employed to heat a circular See also:row of crucibles See also:standing on a shelf along the See also:wall of the furnace. This system, however, has long been abandoned. The modern processes may be primarily divided into two See also:groups according to the nature of the See also:vessel in which the operation is effected. These distilling vessels are called retorts if they are supported only at the ends, and the furnace using them is termed a Belgian furnace. If they are supported at intervals along a See also:flat See also:side, they are called muffles, and the furnace is known as a Silesian furnace. Various combinations and modifications of these two types of furnace have given rise to distinctive names, and as each system has its advantages and disadvantages See also:local conditions determine which is the better. In the Belgian process the reduction and distillation are carried out in cylindrical or elliptical retorts of See also:fire-clay, from 3 ft. 3 in. to 4 ft. 9 in. long and 6 to Io in. See also:internal See also:diameter. Some See also:forty-six or more retorts, arranged in parallel See also:horizontal rows, are heated in one furnace. The furnaces are square and open in front, to allow the outlet ends of the retorts to project; they are grouped together by fours; and their several chimneys are within the same enclosure. Each See also:retort is provided with two adapters, namely, a conical pipe of fire-clay, about 15 in. long, which fits into the retort end, and a conical See also:tube of See also:sheet iron, which fits over the end of the fire-clay pipe, and which at its outlet end is only about an See also:inch wide. To start a new furnace, the front side is closed provisionally by a See also:brick wall, a fire lighted inside, and the temperature raised very gradually to a white heat. After four days' heating the provisional front wall is removed piecemeal, and the retorts, after having been heated to redness, are inserted in corresponding sets. The See also:charge of the retorts consists of a mixture of i 100 lb of roasted calamine and 550 lb of dry powdered See also:coal per furnace. A newly started furnace, however, is used for a time with smaller charges. Supposing the last of these preliminary distillations to have been completed, the residues See also:left in the retorts are removed, and the retorts, as they See also:lie in the hot furnace, are charged by means of semi-cylindrical shovels, and their adapters put on. The charging operation being completed, the temperature is raised, and as a consequence an See also:evolution of carbon monoxide soon begins, and becomes visible by the gas bursting out into the characteristic See also:blue See also:flame. After a time the flame becomes dazzling white, showing that zinc vapour is beginning to See also:escape. The iron adapters are now slipped on, and left on for two See also:hours, when, as a See also:matter of experience, a considerable amount of zinc has gone out of the retort, the greater part into the fire-clay adapter, the See also:rest into the iron See also:cone. The former contains a mixture of semi-solid and molten metal, which is raked out into iron ladles and See also:cast into plates of 66 to 97 lb weight, to be sold as " spelter." The contents of the iron recipient consist of a powdery mixture of oxide and metal, which is added to the next charge, except what is put aside to be sold as " zinc dust." This dust may amount to to per cent. of the See also:total production. As soon as the adapters have been cleaeed of their contents, they are replaced, and again left to themselves for two hours, to be once more emptied and replaced, &c. The complete exhaustion of the charge of a furnace takes about eleven hours. In the Silesian process the distillation is conducted in specially constructed muffles of a prismatic shape arched above, which are arranged in two parallel rows within a See also:low-vaulted furnace, similar to the pots in a glass furnace. As a See also:rule every furnace accommodates ten muffles. Through an orifice in the outlet pipe (which is closed during the distillation by a loose plug) a hot iron See also:rod can be introduced from time to time to clear away any solid zinc that may threaten to obstruct it. As soon as the outlet pipe has become sufficiently hot the zinc flows through it and collects in conveniently placed receptacles. About six or eight hours after starting the distillation is in full See also:swing, and in twenty-four hours it is completed. A fresh charge is then put in at once, the muffles being cleared only after three successive distillations. The distillate consists of a See also:conglomerate of drops (" drop zinc "). It is fused up in iron basins lined with clay, and cast out into the customary form of cakes. The chief improvements in the plant of these processes are concerned with the manufacture of the retorts or muffles, and especially with the introduction of gas-firing. Even a See also:machine of See also:simple type, like the See also:ordinary drain-pipe machine, in which the retorts are made by forcing the plastic clay mixture through a See also:die, may result in greater See also:economy and uniformity than is possible when retorts are made by See also:hand. When See also:hydraulic pressure to the amount of 2000 to 3000 lb per square inch is applied, the saving is unquestioned, since less time is required to dry the pressed retort, its life in the furnaces is longer, its absorption of zinc is less, and the loss of zinc by passage through its walls in the form of vapour is reduced. Three modes of gas-firing are to be noticed, each of which is adapted to See also:special local conditions. (a) The gas is made from the fuel in a detached fireplace and conducted while hot into the See also:combustion chamber of the furnace, and the air for complete combustion is heated by the products of combustion on their way to the See also:chimney. (b) Both the producer gas and the air are heated before they enter the combustion chamber, as in the See also:Siemens system of regenerative firing. (c) Natural gas is piped to the furnace, where it meets air heated by the chimney gases. The See also:primary advantages of gas-firing are that less fuel is required, that there is better See also:control of the heat in the furnace, and that larger and more accessible furnaces can be built. In Silesia the introduction of gas-firing has led to the use of furnaces containing eighty muffles. In the United States, Belgian furnaces of type (a) are built to contain 864 retorts; of type (b), to contain 300 to 400 retorts; and of type (c), prefer-ably about 600 retorts. The use of gas-fired furnaces greatly simplifies See also:manual labour. On a See also:direct-fired furnace at least one See also:man, the brigadier, must be an See also:expert in all the operations involved; but with a gas furnace a See also:division of labour is possible. One man who understands the use of gaseous fuel can regulate the heat of a thousand or more retorts. The men who charge and empty the retorts, those who draw and cast the metal, and those who keep the furnace in repair, need not know anything about the making or using of gas, and the men who make the gas need not know anything about a zinc furnace. Again, in direct-fired furnaces there are commonly seven or eight rows of retorts, one above another, so that to serve the upper rows the workman must stand upon a table, where he is exposed to the full heat of the furnace and requires a helper to wait upon him. With gas-firing the retorts can be arranged in four horizontal rows, all within reach of a man on the furnace-See also:room See also:floor. Furthermore, with the large furnaces which gas-firing makes possible mechanical appliances may be substituted for manual labour in many operations, such as removing and replacing broken retorts, mixing and conveying the charge, See also:drawing and casting the metal, charging and emptying the retorts, and removing the residues and products. Refining.—The specific effects of different impurities on the See also:physical properties of zinc have only been imperfectly studied. Fortunately, however, the small amounts of any of them that are likely to be found in commercial zinc are not for most purposes very deleterious. It is generally recognized that the purest ores produce the purest metal. Grades of commercial zinc are usually based on selected ores, and brands, when they mean anything, usually mean that the metal is made from certain ores. Chemical control of the metal See also:purchased is not nearly as See also:common as it should be, and the refining of zinc is at best an imperfect operation. To obtain the metal chemically pure a specially prepared pure oxide or See also:salt of zinc is distilled. A redistilled zinc, from an ordinarily pure commercial zinc, is often called chemically pure, but redistillation is seldom practised except for the recovery of zinc from galvanizer's dross and from the skimmings and bottoms of the melting furnaces of zinc See also:rolling See also:mills. The only other method of refining is by oxidizing and settling. A See also:bath, even of very impure zinc, is allowed to stand at about the temperature of the melting-point of the metal for forty-eight or more hours, where-upon the more easily oxidizable impurities can be largely removed in the dross at the See also:top, the heavier metals such as See also:lead and iron settling towards the bottom. This method is rarely practised except by the rollers of zinc. A certain amount of refined zinc can be dipped from the furnace; a further amount, nearly free from iron, can be liquated out of the ingots cast from the bottom of the bath in a subsequent slow remelting, and it is sometimes possible to eliminate a zinciferous lead which collects in the sump of the furnace. Owing to the fact that at temperatures between its melting and, boiling point zinc has a strong See also:affinity for iron, it is often contaminated by the scraper while being See also:drawn from the See also:condenser, as is shown by the fact that the scraper wears away rapidly. As each retort in a furnace is in all essentials a See also:separate crucible, and as the metal from only a few of them goes into a single See also:ingot, there can be no uniformity either in the ingots made from the same furnace during a See also:day's run or in those made from several furnaces treating the same ore. Some brassfounders break from a single ingot the quantity of zinc required to produce the amount of brass they wish to compound in one crucible, but when perfect uniformity is desired the importance of remelting the zinc on a large See also:scale cannot be too strongly emphasized. Electrolytic Separation of Zinc.—The deposition of pure zinc is beset with many difficulties. Zinc being more electro-See also:positive even than See also:nickel, all the heavy metals must be removed before its deposition is attempted. Moreover, unless the conditions are closely watched, it is liable to be thrown down in a spongy form. M. Kiliani found that the sponge was produced chiefly when a weak See also:solution, or a low current-See also:density, was used, and that See also:hydrogen was usually evolved simultaneously; See also:sound deposits resulted from the use of a current-density of 200 amperes, or more, per sq. ft., and strong solutions. The cause of the spongy deposit is variously explained, some (Siemens and Halske) ascribing it to the existence of a compound of zinc and hydrogen, and others, among whom are G. Nahnsen, F. Mylius and A. Fromm, F. Foerster and W. Borchers, trace it to the presence of oxide, produced, for example, either by the use of a solution containing a trace of basic salt of zinc (to prevent which the bath should be kept just—almost imperceptibly—acid), or by the presence of a more electro-negative metal, which, being co-deposited, sets up local action at the expense of the zinc. Many processes have been patented, the ore being acted upon by acid, and the resulting solution treated, by either chemical or electrolytic means, for the successive removal of the other heavy metals. The pure solution of zinc is then electrolysed. E. A. Ashcroft patented a process of dealing with complex ores of the well-known Broken See also: C. Hoepfner has patented several processes, in one of which (No. 13,336 of 1894) a rapidly rotating cathode is used in a chloride solution, a porous See also:partition separating the tank into anode and cathode compartments, and the See also:chlorine generated by electrolysis at the anode being recovered. Hoepfner's processes have been employed both in England and in Germany. Nahnsen's process, with an electrolyte containing See also:alkali-metal sulphate and zinc sulphate, has been used in Germany, and a process invented by Dieffenbach has also been tried in that See also:country. Siemens and Halske have proposed the addition of oxidizing agents such as free See also:halogens, to prevent the formation of zinc hydride, to which they attribute the formation of zinc-sponge. Borchers and others deposit zinc from the fused chloride. In Borchers' process the chloride is heated partly by See also:external firing, partly by the heat generated owing to the use of a current-density of 90 to 100 amperes per sq. ft. PROPERTIES Zinc is a bluish-white metal, showing a high lustre when freshly fractured. It fuses at 415° C. and under ordinary atmospheric pressure boils at Io4o° C. Its vapour density shows that it is monatomic. The molten metal on cooling deposits crystals belonging to the hexagonal system, and freezes into a compact crystal-line solid, which may be brittle or ductile according to circumstances. If zinc be cast into a See also:mould at a red heat, the ingot produced is laminar and brittle; if cast at just the fusing-point, it is granular and sufficiently ductile to be rolled into sheet at the ordinary temperature. According to some authorities, pure zinc always yields ductile ingots. Commercial " spelter " always breaks under the See also:hammer; but at 100° to 150° C. it is susceptible of being rolled out into even a very thin sheet. Such a sheet, if once produced, remains flexible when See also:cold. At about 200° C., the metal becomes so brittle that it can be pounded in a See also:mortar. The specific gravity of zinc cannot be expected to be perfectly constant; according to See also:Karsten, that of pure ingot is 6.915, and rises to 7.191 after rolling. The coefficient of linear expansion is 0.002,905 for See also:loo° from o° upwards (See also:Fizeau). The specific heat is 0.09555 (See also:Regnault). Compact zinc is bluish white; it does not tarnish much in the air. It is fairly soft, and clogs the See also:file. If zinc be heated to near its boiling-point, it catches fire and See also:burns with a brilliant light into its powdery white oxide, which forms a reek in the air (lana philosophica, " philosopher's See also:wool "). Boiling See also:water attacks it appreciably, but slightly, with evolution of hydrogen and formation of the hydroxide, Zn(OH)2. A rod bf perfectly pure zinc, when immersed in dilute sulphuric acid, is so very slowly attacked that there is no visible evolution of gas; but, if a piece of See also:platinum, copper or other more electro-positive metal be brought into contact with the zinc, it dissolves readily, with evolution of hydrogen and formation of the sulphate. The ordinary impure metal dissolves at once, the more readily the less pure it is. Cold dilute nitric acid dissolves zinc as nitrate, with evolution of nitrous oxide. At higher temperatures, or with stronger acid, nitric oxide, NO, is produced besides or instead of nitrous. Zinc is also soluble in soda and potash solutions, but not in See also:ammonia. Applications.—Zinc is largely used for " galvanizing" iron, sheets of clean iron being immersed in a bath of the molten metal and then removed, so that a coat of zinc remains on the iron, which is thereby protected from atmospheric corrosion. It is also a constituent of many valuable See also:alloys; brass, Muntz-metal, pinchbeck, tombac, are examples. In technological See also:chemistry it finds application as a reducing See also:agent, e.g. in the production of See also:aniline from See also:nitrobenzene, but the use of iron is generally preferable in view of the cheapness of this metal. COMPOUNDS Zinc forms only one oxide, ZnO, from which is derived a well-characterized See also:series of salts. It is chemically related to cadmium and See also:mercury, the resemblance to cadmium being especially well marked; one distinction is that zinc is less basigenic. Zinc is capable of isomorphously replacing many of the bivalent metals—magnesium, manganese, iron, nickel, See also:cobalt and cadmium. Zinc oxide, ZnO, is maufactured for paint by two processes—directly from the ore mixed with coal by volatilization on a See also:grate, as in the Wetherill oxide process, and by oxidizing the vapour given off by a boiling bath of zinc metal. The oxide made by the latter method has generally a better colour, a finer texture, and a greater covering See also:power. It is also manufactured by the latter process from the metallic zinc liquated out of galvanizer's dross. It is an infusible solid, which is intensely yellow at a red heat, but on cooling becomes white. This at least is true of the oxide produced from the metal by combustion; that produced from the carbonate, if once made yellow at a red heat, retains a yellow shade permanently. By heating the nitrate it is obtained as hemimorphous pyramids belonging to the hexagonal system; and by heating the chloride in a current of See also:steam as hexagonal prisms. It is insoluble in water; it dissolves readily in all aqueous acids, with formation of salts. It also dissolves in aqueous See also:caustic alkalis, including ammonia, forming " zincates " [e.g. Zn(OK)2]. Zinc oxide is used in the arts as a white pigment (zinc white) ; it has not by any means the covering power of white lead, but offers the advantages of being non-poisonous and of not becoming discoloured in sulphuretted hydrogen. It is used also in See also:medicine. Zinc hydroxide, Zn (OH)2, is prepared as a gelatinous precipitate by adding a solution of any zinc salt to caustic potash. The alkali must be free from carbonate and an excess of it must be avoided, otherwise the See also:hydrate redissolves. It is a white See also:powder, and is insoluble in water. To acids and to alkalis it behaves like the oxide, but dissolves more readily. Zinc chloride, ZnC12, is produced by heating the metal in dry chlorine gas, when it distils over as a white translucent See also:mass, fusing at 250° and boiling at about 40o°. Its vapour-density at 900° C. corresponds to ZnCl2• It is extremely hygroscopic and is used in synthetical organic chemistry as a condensing agent. It dissolves in a fraction of its weight of even cold water, forming a syrupy solution. A solution of zinc chloride is easily produced from the metal and hydrochloric acid; it cannot be evaporated to dryness without considerable decomposition of the hydrated salt into oxychloride and hydrochloric acid, but it may be crystallized as ZnCl2•H2O. A concentrated solution of zinc chloride converts See also:starch, See also:cellulose and a See also:great many other organic bodies into soluble compounds; hence the application of the fused salt as a caustic in See also:surgery and the impossibility of filtering a strong ZnC12 solution through See also:paper (see CELLULOSE). At a boiling heat, zinc chloride dissolves in any proportion of water, and highly concentrated solutions, of course, See also:boil at high temperatures; hence they afford a convenient See also:medium for the See also:maintenance of high temperatures. Zinc chloride solution readily dissolves the oxide with the formation of oxychlorides, some of which are used as See also:pigments, cements and for filling See also:teeth in See also:dentistry. A solution of the oxide in the chloride has the See also:property of dissolving See also:silk, and hence is employed for removing this fibre from wool. Zinc bromide, ZnBr2, and Zinc iodide, Zn12, are deliquescent solids formed by the direct union of their elements. With ammonia and alkaline bromides and iodides See also:double salts are formed. Zinc sulphide, ZnS, occurs in nature as blende (q.v.), and is artificially obtained as a white precipitate by passing sulphuretted hydrogen into a neutral solution of a zinc salt. It dissolves in mineral acids, but is insoluble in acetic acid. Zinc sulphate, ZnSO4+7H20, or white See also:vitriol, i4. prepared by dissolving the metal in dilute sulphuric acid. If care be taken to keep the zinc in excess, the solution will be free from all See also:foreign metals except iron and perhaps manganese. Both are easily removed by passing chlorine through the cold solution, to produce ferric and manganic salt, and then digesting the liquid with a washed precipitate of basic carbonate, produced from a small portion of the solution by means of See also:sodium carbonate. The iron and manganese are precipitated as hydroxides, and are filtered off. The filtrate is acidified with a little sulphuric acid and evaporated to See also:crystallization. The salt crystallizes out on cooling with 7 molecules of water, forming colourless orthorhombic prisms, usually small and See also:needle-shaped. They are permanent in the air. According to Poggiale, loo parts of water dissolve respectively of (7H20) salt, 115.2 parts at-0°, and 653.6 parts at 1oo°. At 1 oo° C. the crystals lose 6 of their molecules of water; the remaining See also:molecule goes off at 250°, a temperature which lies See also:close to that at which the salt begins to decompose. The anhydrous salt, when exposed to a red heat, breaks up into oxide, sulphur dioxide and See also:oxygen. An impure form of the salt is prepared by roasting blende at a low temperature. In the arts it is employed in the preparation of varnishes, and as a See also:mordant for the production of See also:colours on See also:calico. A See also:green pigment known as Rinmann's green is prepared by mixing loo parts of zinc vitriol with 2.5 parts of cobalt nitrate and heating the mixture to redness, to produce a compound of the two oxides. Zinc sulphate,'like magnesium sulphate, unites with the sulphates of the See also:potassium metals and of ammonium into crystalline double salts, ZnSO4•R2SO4+6H2O, isomorphous with one another and the magnesium salts. Zinc carbonate, ZnCO3, occurs in nature as the mineral calamine (q.v.), but has never been prepared artificially, basic carbonates, ZnCO3.xZn(OH)2, where x is variable, being obtained by precipitating a solution of the sulphate or chloride with sodium carbonate. To obtain a product free of Cl or SO4, there must be an excess of alkali and the zinc salt must be poured into the hot solution of the carbonate. The precipitate, even after exhaustive washing with hot water, still contains a trace of alkali; but from the oxide, prepared from it by ignition, the alkali can be washed away. The basic carbonate is used as a pigment. Of zinc See also:phosphates we See also:notice the minerals hopeite, Zn.3 (PO4)2.4H2O, and tarbuttite, Zn3(PO4)2.Zn(OH)2, both found in See also:Rhodesia. See also:Analysis.—From neutral solutions of its salts zinc is precipitated by sulphuretted hydrogen as sulphide, ZnS—a white precipitate, soluble, but by no means readily, in dilute mineral acids, but insoluble in acetic acid. In the case of acetate the precipitation is quite complete; from a sulphate or chloride solution the greater part of the metal goes into the precipitate; in the presence of a sufficiency of free HCl the metal remains dissolved; sulphide of ammonium precipitates the metal completely, even in the presence of ammonium salts and free ammonia. The precipitate, when heated, passes into oxide, which is yellow in the heat and white after cooling; and, if it be moistened with cobalt nitrate solution and re-heated, it exhibits a green colour after cooling. Zinc may be quantitatively estimated by precipitating as basic carbonate, which is dried and ignited to zinc oxide. It may also be precipitated as zinc ammonium phosphate, NH4ZnPO4, which is weighed on a See also:filter tared at loo . Volumetric methods have also been devised. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] ZIMMERMANN, JOHANN GEORG, RITTER VON |
[next] ZINCITE |