Zinc is a metallic element with symbol Zn, atomic
number 30, atomic weight 65.37.
It is present within the 2nd group and more precisely in the subgroup B of the periodic system, in its compounds always has an oxidation number of +2.
The melting temperature is 419.5°C and the boiling temperature is reached AT 907°C.
Its mechanical properties allow extreme resistance and excellent hardness (greater than tin and lead).
Used in alloys since ancient times (especially with copper and tin), it has been known and used in the pure state in fairly recent times, around the middle of 1700.
Its use spread when the possibility to hot laminate it was discovered.
White in colour tending to blue, zinc does not exist free in nature, the most important minerals containing it are: calamine (or zinc silicate) blende (or zinc sulphide) smithsonite (or zinc carbonate) hereinafter also zincite (or zinc oxide), franklinite (or zinc spinel) but less important than the first three listed.
The deposits of these minerals are present, mostly in the United States, Latin America, Australia, central and northern Europe and even in Italy largely in the regions of Sardinia and Lombardy.
EXTRACTION OF ZINC
There are two processes to extract zinc from minerals in which it is contained, the thermal process and the electrolytic process.
The thermal process is the most widely used today and the oldest.
The minerals, through a process defined as "roasting", are transformed into zinc oxide, the latter is subsequently transformed, by distillation in metallic zinc due to the high temperature of operation.
This process requires after a process defined as Refining, rarely, in fact, with the thermal process the zinc reaches a purity of 98%, in this stage particles of lead, iron, cadmium, arsenic, antimony and sulfur are still present .
To purify the zinc the metal temperature is taken back to about 430°C for a period of time, as a result of this additional fusion, the lead will settle to the bottom of the tank while the other elements will place themselves on the surface.
By appropriate work of cleaning, zinc, now pure to 99.9%, is cast into bars that are laminated while the metal is still hot.
The electrolytic process consists in submitting to the detergent and whitening action (leach) with diluted sulfuric acid, the product which is obtained through Roasting at a low temperature of the concentrates of sphalerite.
This operation is carried out within a series of tanks (Pachuca vats) inside of which air is blown in order to maintain the material under stirring.
The solutions are purified from other metals aluminum, iron, silica which precipitate as hydroxides by adjusting the pH to 5-5.5, copper, cadmium and nickel precipitate with zinc dust cementation, while cobalt is removed by precipitation with alpha-naphthol-beta-naphthol; arsenic and antimony are often included or can be absorbed in the same precipitates.
From the purified solution following filtration is performed the electrolysis process with a concentration of 75-80 g / l of zinc.
For the anode is used lead, while for the cathode is employed the aluminum; to the terminals there is a voltage of 3.5-4V.
Zinc, if the solutions were suitably purified, is deposited on the bottom, up to 99.9% pure.