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The chief ore zinc is sphalerite (ZnS), also known as the zinc blende, which usually occurs in association with galena (PbS) and smaller quantities of other metallic sulphides. Concentrates with more than 50% zinc are produced by flotation. From these the zinc can be obtained by various thermal reduction processes or by leaching and electrolysis. In every case the process must be preceded by complete roasting to convert zinc sulphide into zinc oxide and thereby make it leachable or reducible with carbon.

The name of the metal zinc is unusual and, while vague in origin, was probably first used by Paracelsus, a Swiss-born German chemist, who referred to the metal as Zincum, in the 16th century. These words in German apparently mean "tooth-like, pointed or jagged part" and, as zinc metallic crystals are needle-like, the derivation appears plausible. Zinc mines of Zawar, near Udaipur, Rajasthan, were active during 1300–1000 BC. There are references of medicinal uses of zinc in the Charaka Samhita (300 BC).

The Rasaratna Samuccaya (800 AD) explains the existence of two types of ores for zinc metal, one of which is ideal for metal extraction while the other is used for medicinal purpose. Because of the low boiling point and high chemical reactivity of this metal (isolated zinc would tend to go up the chimney rather than be captured), the true nature of this metal was not understood in ancient times.

From low-grade zinc ores and from intermediate products containing zinc, such as flue dusts from lead and copper smelting, it is possible to obtain zinc oxide by the so-called rotary process; the material containing zinc is heated under reducing conditions in a long rotary kiln i.e. a tubular cylindrical furnace which is inclined and revolves on its longitudinal axis; the raw material is fed in at the upper end and gradually makes its way down to the lower end, acquiring a progressively higher temperature on the way.

In the kiln the zinc is volatilized; in the upper part the zinc vapor is burned to zinc oxide, which leaves the kiln along with waste gases and is collected in a bag-filter plant. Zinc production by dry processing presents particular difficulties because zinc has a low boiling point (906 oC) and therefore occurs only in gaseous form at the temperature necessary for effecting the reduction (1300oC).

For this reason reduction has to be carried out in closed vessels or furnaces in which the zinc vapor can be condensed in the absence of air. For thermal reduction the concentrate is roasted a treatment that may be carried out in two stages multiple-hearth furnace followed by sintering on a traveling grate or in a single-stage operation in which a proportion of the roasted material is fed back to the sintering machine. Sintering is necessary for transforming the material into suitable lumps to allow air to flow through it during the subsequent reduction process.

Reduction of the zinc oxide can be done by various methods. In the so-called standard process, reduction is effected in horizontal retorts in a retort furnace or distilling furnace (Fig.2), which comprises a lower part containing regenerative chambers for preheating the gas and combustion air and an upper part in which retorts, arranged in tiers one above the other, are heated by the hot-flame gases. A retort of this kind is a rectangular distilling vessel, about 6or 7ft. long and about 1 ft. square in cross section. It is made of fireclay and has only a short service life, having to be renewed every four to six weeks.


The zinc vapor escapes from the retorts and is collected in the condensers as liquid metal. At the end of about 20 hours the process has been completed; the liquid zinc and zinc dust are then removed from the condensers, and the residual matter is removed from the retorts by special machines. Cleaning out the retorts and recharging them takes about 4 hours, so that the whole cycle can be repeated every 24 hours. In a more recent development of the process a single large condensing chamber is used instead of individual condensers.


In the New Jersey process (Fig.4) reduction is effected in large vertical retorts about 14m (45 ft) high and of rectangular cross section, lined with silicon-carbide bricks in the hottest parts. The advantage over the horizontal retort method is that reduction can be performed as a continuous operation. The charge consist of briquettes made of a mixture of zinc oxide material (roasted blende) and bituminous coal. The briquettes are fed automatically to the resort, the residual matter being extracted by a screw conveyor at the bottom. The mixture of carbon monoxide and zinc vapor is discharged from the top of the retort into a condenser, where the metal is precipitated in liquid form.

There have been many attempts to utilize electricity for the smelting of zinc. The St. Joseph Lead Co. (U.S.A) has developed a successful method of reducing zinc in an arc furnace (Fig.5). The process is similar in principle to the retort method, except that the heat is now supplied by the electric arc. Another fairly recent development is the method employed at Avonmouth (Great Britain), which has significantly affected the metallurgical processing of lead as well as zinc.


In this method, known as the Imperial Smelting process, the two melts are produced simultaneously from the oxides of zinc and lead in a shaft furnace. The process is especially valuable for dealing with ores in which sphalerite and galena occur in intimate association with each other. The furnace is charged with coke and a mixture of roasted lead and zinc ores, prepared by a pressure sintering process. In the furnace, which is really a form of blast furnace, the lead oxide is reduced to molten metallic lead, which collects at the bottom.

The zinc oxide is likewise reduced and forms zinc vapor, which is extracted at the top of the furnace along with the combustion gases. The vapor is passed to a condenser in which the cooling medium is molten lead, in which the zinc dissolves. The zinc-in-lead solution is then passed into a separator in which on cooling, a layer of liquid zinc forms a top of the lead (this separation is due to the fact that the solubility of zinc in lead diminishes at the lower temperature). The lead is returned to the condenser, and the zinc is further processed by refining.

The lower part of the first column is heated. Impure zinc is fed continuously into the top of the column and is vaporized as it flows down through the heated trays. After further purification by refluxing in the upper part of the column, the zinc vapor (still containing cadmium but free of other impurities) is passed to a condenser, whence it is fed to the top of the second column, in which all the cadmium is driven off. Zinc of 99.995% purity is condensed and drawn from the bottom. Another widely used method of zinc production is by the leaching of roasted zinc concentrates with acid and then depositing the zinc by electrolysis from the solution thus obtained.