Introduction:- ZINC

Centuries before zinc was discovered in the metallic form, its ores were used for making brass and zinc compounds were used for healing wounds and sore eyes. The Romans produced brass in the time of Augustus (20 B.C. – 14 A.D.). By 1374, zinc was recognized in India as a new metal and at Zawar, India, both zinc metal and zinc oxide were produced from the 12th to the 16th century. From India, zinc manufacture moved to China in the 17the century. Zinc was recognized as a separate metal in Europe in 1546. In 1743, the first European zinc smelter was established at Bristol in the United Kingdom.
Zinc was named by the Swiss alchemist Theophrastus Bombastus von Hohenheim (Paracelsus, 1493-1541), who coined the new Latin word zincum from antecedents.

Proprieties of Zinc

o Zinc is a bluish-grey metal covered by a protective transparent layer of basic carbonate in air.

o A sheet of zinc looks very much like a sheet of aluminium, but it is more than twice as heavy, and does not bend easily.

o Zinc is not very ductile or malleable, especially when pure.

o Its density is 7.14 g/cc, electrical resistivity 6.16 ??-cm, heat capacity 0.0925 cal/g-K, and heat conductivity 0.268 cal/cm-s-K. Its coefficient of linear expansion is 40.0 x 10-6 per K.

o Zinc melts at 419.5°C and boils at 907°C. The heat of fusion is 24.09 cal/g. In the cast form, its tensile strength is only 4-12 ksi, but the cold work of rolling gives 28-36 ksi. Hard-drawn zinc has strength of about 10 ksi. The Young’s modulus is 12.4 x 106 psi. Zinc, at Mohs 2.5, is harder than tin or cadmium. Its crystal form is hexagonal close packed, with a = 0.266 nm, c = 0.494 nm. The ionic radius of Zn++ is 0.074 nm. The ionization potentials of zinc are 9.36V and 17.89V.

Production Process of Zinc

The raw material used for the production of zinc is zinc concentrate, which is the result of a flotation process after the ore has been mined and milled. The zinc ore contains 1-15% zinc whereas the concentrate typically contains approx. 55% zinc, 6.5% iron and 32% sulphur together with other elements at much lower levels. The process begins with the roasting of the concentrate. At a temperature of around 950°C, oxidisation of the zinc, iron and sulphur occurs. The sulphur is collected as SO2 and is used to make sulphuric acid (H2SO4) – a commercial by-product.

Zinc Mining

80% of zinc mines are underground, 8% are of the open pit type and the remainder is a combination of both. However, in terms of production volume open pit mines account for as much as 15%, underground mines produce 64% and 21% of mine production comes from the combined underground and open pit mining. Rarely is the ore, as mined, rich enough to be used directly by smelters; it needs to be concentrated. Zinc ores contain 5 -15% zinc. To concentrate the ore it is first crushed and then ground to enable optimal separation from the other minerals. Typically, a zinc concentrate contains about 55% of zinc with some copper, lead and iron. Zinc concentration is usually done at the mine site to keep transport costs to smelters as low as possible.

Roasting & Sintering

Over 95% of the world’s zinc is produced from zinc blende (ZnS). Apart from zinc the concentrate contains some 25-30% or more sulphur as well as different amounts of iron, lead and silver and other minerals. Before metallic zinc can be recovered, by using either hydrometallurgical or pyrometallurgical techniques, sulphur in the concentrate must be removed. This is done by roasting or sintering. The concentrate is brought to a temperature of more than 900°C where zinc sulphide (ZnS) converts into the more active zinc oxide (ZnO). At the same time sulphur reacts with oxygen giving out sulphur dioxide which subsequently is converted to sulphuric acid – an important commercial by-product.

The Hydrometallurgical Process

In a leaching stage the zinc oxide is separated from the other calcines. Sulphuric acid is used to do this. The zinc content dissolves whereas iron precipitates and lead and silver remain undissolved. However, the dissolved solution contains some impurities, which need to be eliminated in order to obtain a high-purity zinc product at the end of the production process. Purification is mainly done by adding zinc dust to the solution. As all the elements to be removed lie below zinc in the electrochemical series they can be precipitated by cementation. The thus obtained purified solution passes an electrolytic process where the purified solution is electrolyzed between lead alloy anodes and aluminium cathodes. An electrical current is circulating through the electrolte by applying an electrical difference of 3.3 – 3.5 volts between the anode and cathode causing the zinc to deposit on the aluminium cathodes in high purity. The deposited zinc is stripped off, dried, melted and cast into ingots. The zinc ingots may have different grades: High Grade (HG) 99.95 % and Special High Grade (SHG) 99.99% of zinc. Today over 90% zinc is produced hydrometallurgically in electrolytic plants.

The Pyrometallurgical Process

The Imperial Smelting Process has been the most important pyrometallurgical process. It allows simultaneous production of zinc and lead metals – roughly 1 ton of lead for every 2 tons of zinc. It is particularly indicated for treating concentrates with a significant amount of lead. The Imperial Smelting process is based on the reduction of zinc and lead into metal with carbon in a specially designed Imperial Smelting furnace. Pre-heated air is blown from below in the shaft furnace. The sinter is charged together with the pre-heated coke at the top of the furnace. Temperatures range from 1000°C at the top to 1500°C or more in the center of the furnace. The coke is converted into carbon monoxide, which provides the means to reduce zinc and lead oxides to metallic zinc and lead. The lead, which is below its boiling point, flows from the bottom of the blast furnace, carrying copper, silver and gold with it. Zinc evaporates and passes out of the furnace near the top along with other gases. To avoid that it oxidizes back to zinc oxide the zinc vapor is rapidly quenched and dissolved in a spay of molten lead in a condenser (lead splash condenser). By cooling the lead, crude zinc is released and is separated. The lead returns to the condensing process for another cycle of dissolving and then releasing more zinc.
The IS process is an energy-intensive process and thus became very expensive following the rise of energy prices in recent years. This and the lower production of bulk concentrates containing significant amounts of lead led to abandoning more and more the Imperial Smelting process. Today, Imperial Smelting furnaces are only in operation in Japan, China and Poland. The major difference of the hydrometallurgical process and the Imperial Smelting process is that the first produce very pure zinc directly whereas the latter produces lower grade zinc that still contains significant impurities that have to be removed by thermal refining in the zinc refinery.