Currently, about 34 of the coils produced by hot strip mill in the world are provided for continuous strip pickling units, of which 98 percent are horizontal (horizontal) continuous pickling units.Therefore, from the perspective of the large number and profound significance of pickling unit in treating strip steel, it is necessary to study the pickling mechanism and rule of pickling unit so that the unit can exert its maximum production potential.
Pickling is the process of removing the oxidized metal on the surface of a metal by chemical means.According to its production methods, pickling is usually divided into acid method, alkali-acid method, hydride method, electrolysis method and so on.However, most of the continuous pickling machines use acid or hydrochloric acid pickling methods.
In the continuous strip pickling unit, hydrochloric acid is usually listed as one of the three strong acids in chemical industry, together with sulfuric acid and nitric acid.
In the chemical industry, hydrochloric acid is used to burn pure hydrogen in chlorine gas to produce hydrogen chloride, which is then absorbed by water.The chemical equation is :
Pure hydrochloric acid is a colorless and transparent liquid with pungent odor. Concentrated hydrochloric acid contains about 37% of radon chloride, with a proportion of about 1.19. Industrial hydrochloric acid is yellow due to the presence of impurities (mainly FeCl).Concentrated hydrochloric acid in the air often "smoke," because of the hydrogen chloride out of the hydrochloric acid met the air water condensation into small drops of hydrochloric acid.At a lower boiling point, such as hydrochloric acid, the vaporization of a molecular acid, usually called volatile acid, becomes more pronounced at room temperature.
Hydrochloric acid has all the chemical properties of acid. It can react with metal or metal oxides to form salt, hydrogen or water.Such as:
Fe + 2 hci - = FeCl2 + H2
FeO + 2 hci -- FeCl + HO
Hydrochloric acid also has certain corroding effect on the skin or some fabrics should be careful when using, do not put hydrochloric acid on the skin or clothes.
Due to volatile hydrochloric acid, volatile HC1 gas to have larger, metal and building damage effect, such as the content of HCI in the air to one over one million, which is 1 PPM, can make smooth metal dimming, therefore, or the use of hydrochloric acid in the hydrochloric acid pickling countries place must be strictly control the content of HCI in the air, no more than 5 mg/m3.