CHEMISTRY : Metal Cation Identification

Information on ZINC

1. General Information


2. Occurence Uses and Properties


3. History of the Metal


4. Compounds
5. Back to Main Metal List

Metallic zinc appeared much later in history than the other common metals. Copper, lead, tin, and iron can be obtained as the molten metals by heating their oxide ores with charcoal (carbon), a process called reduction, in shaft furnaces, which were developed quite early in history. Zinc oxide, however, cannot be reduced by carbon until temperatures are reached well above the relatively low boiling point of the metal (907 C). Thus, the furnaces developed to smelt the other metals could not produce zinc. Small quantities of metallic zinc can sometimes be found in the flues of lead blast furnaces. There is some evidence that the Greeks knew of the existence of zinc and called it pseudargyras, or "false silver," but they had no method of producing it in quantity.

The Romans as early as 200 BC produced considerable quantities of brass, an alloy of zinc and copper, by heating in crucibles a mixture of zinc oxide and charcoal covered with lumps of metallic copper. The zinc oxide was reduced in the lower part of the crucible. Zinc vapour was formed and dissolved in the copper to form brass. At the end of the process the temperature was raised to melt the brass for casting into ingots. The realization that to make zinc it was necessary to produce the metal as a vapour and then condense it seems first to have been reached in India in the 14th century. In the West this principle was first applied in England in 1743. At the end of the 18th century in Belgium and Poland improvements were made in the furnace, and the process remained unchanged until an electrolytic process was developed in 1917. At the end of the 1920s a radical advance was made in the United States by developing a continuous retort process, and during the 1930s an electrothermic process was designed for producing zinc continuously. A development of the 1960s was the zinc-lead blast furnace, in which rapid quenching of the gases is a key principle. Zinc production processes are treated in detail in zinc processing .

The separation of metallic zinc from its ores by pyrometallurgy is much more difficult than with other common metals, such as copper, lead, and iron, because the reduction of zinc oxide by carbon (C) proceeds spontaneously only above the zinc boiling point of 907 C (1,665 F). Efficient methods of condensing the vapour to liquid metal were not discovered until the 14th century AD. As an alloy constituent, however, zinc was in use well before that time. Brass, an alloy of copper and zinc, was produced by the Romans as early as 200 BC by heating copper, zinc oxide (ZnO), and carbon together. The zinc formed by the reduction of its oxide was absorbed into the copper and did not appear as a separate phase.

Evidence suggests that zinc was first produced in quantity in India and China. At Zawar in Rajasthan, India, the remains of a smelting industry dating from the 14th century have been found. Although no written record exists, the process appears to have involved large numbers of small clay retorts, which were charged with zinc oxide and charcoal, placed in a setting, and heated. The exact method of condensing and collecting the zinc can only be surmised.

Subsequent commercial procedures for zinc production all involved retort processes, the key overall reaction being initiated by external heat and involving the reduction of ZnO to zinc vapour by carbon, which was itself oxidized to carbon monoxide (CO). Important advances were made by William Champion in Bristol, Eng., in the mid-18th century, by Johann Ruberg in Silesia in the late 18th century, and by Jean-Jacques-Daniel Dony in Liège, Belg., in the early 19th century. Belgian-type horizontal retorts were operated in Britain as the main zinc-producing process for about 100 years starting in the mid-19th century. The daily output of each retort was about 40 kilograms (90 pounds), and several hundred retorts were banked together and fired by gas. The process was physically arduous in the extreme and suffered all the disadvantages of small-scale batch operation with high energy and labour costs.

In the late 1920s a continuous vertical-retort process was developed in the United States. The retort was constructed of silicon carbide brick for high heat conductivity, with a rectangular cross section of two metres (six feet) by one-third metre and a height of 11 metres. The charge of roasted sulfide concentrate and anthracite coal was sized, briquetted, and preheated in a coking furnace prior to charging to the heated retort. Zinc vapour, removed with CO at the top of the retort, was condensed in a stirred molten-zinc bath. The output of each retort was about eight tons per day, and a typical plant operated about 20 retorts.

A variant of the vertical retort, known as the electrothermic furnace, was also developed in the United States at about the same time. In this process, heat was supplied through the direct electrical-resistance heating of the coke in the charge.

The most serious disadvantage of the improved retort processes was that they were restricted to ore concentrates with a low iron content, because high iron content in the feed caused plates of iron to form in the retorts. For this reason, zinc production by this means is now obsolete.

Early attempts to devise a blast-furnace process for zinc production failed because of the difficulty of condensing zinc vapour from a gas containing substantial quantities of carbon dioxide. This difficulty was finally overcome in the mid-20th century by the development of the lead-splash condenser, a means of shock-cooling furnace gases and absorbing zinc vapour into solution in molten lead. This allowed the zinc blast furnace to become the main pyrometallurgical means of producing zinc.

The zinc blast furnace should actually be referred to as the zinc-lead blast furnace, since, beginning with the first successful recycling of lead drosses from the condenser, blast-furnace operations evolved to the handling of mixed zinc-lead feed materials up to a ratio of 2:1 zinc to lead.

The major zinc-recovery process, electrolysis, made steady progress after commercial operation commenced around 1915-18. Prior to this, numerous attempts had been made, without success, following a patented method of sulfate electrolysis by the Frenchman Léon Letrange in 1881. The discovery that a high-purity sulfate electrolyte was required led to the eventual success of the process.

In chemical compounds, zinc exhibits almost exclusively a +2 oxidation state. A few zinc(I) compounds have been reported, but never any compounds of zinc(III) or higher.

Zinc oxide, ZnO, is one of the most important zinc compounds. It can be prepared in a state of high purity and in a variety of crystal shapes and sizes by burning zinc vapour in air. Because of its high heat conductivity and capacity, zinc oxide is frequently incorporated into rubber as a heat dissipater. In the crystal of zinc oxide, the lattice (i.e., the orderly structure formed by the ions) is an open one in which the zinc and oxygen ions occupy only 44 percent of the volume. Defects can be created in the lattice by specific treatments such as the introduction of foreign atoms or of zinc atoms in the vacancies of the lattice. Such treatment of zinc oxide crystals produces various electrical, photoelectrical, and CATalytic properties. As a result, zinc oxide is used as a semiconductor in the production of phosphors for television tubes and fluorescent lamps. Its effects on the reactivity of many compounds make it useful as a CATalyst in such operations as the manufacture of synthetic rubber and methanol. It is also used in paints, cosmetics, plastics, pharmaceuticals, and printing inks. Because under the influence of light the electrical conductivity of zinc oxide can be increased many times, it is employed in certain photocopying processes.

Zinc sulfate, ZnSO4, is an intermediate compound in the production of zinc from its ores by the electrolytic process. It is used as a weed killer, in the manufacture of viscose rayon, and in dyeing, in which it functions as a mordant. Zinc chloride, ZnCl2, can be prepared by a direct reaction or by evaporating the aqueous solution formed in various reactions. It is strongly deliquescent (water-absorbing) and is utilized as a drying agent and as a flux. In aqueous form it is used as a wood preservative. Zinc sulfide, ZnS, occurs in nature as the mineral sphalerite and may be prepared by treating solutions of zinc salts with hydrogen sulfide. It was long used as a white pigment but has been gradually replaced by titanium dioxide. Zinc sulfide has luminescent properties when activated by the addition of small quantities of copper, manganese, silver, or arsenic and so has been used in X-ray screens, in luminous dials for clocks and watches, and in fluorescent lights.

atomic number 30 atomic weight 65.39 melting point 420 C (788 F) boiling point 907 C (1,665 F) density 7.133 (25 C) valence 2 electronic config. 2-8-18-2 or (Ar)3d104s2

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