Pickling is one of the effective processes of Cold RolledCoil processing. The theory of pickling is based upon acid wash treatment by 6 to 8% of hydrochloric acid (HCl) to remove the iron scales from the coils. As a result Fe+2, Fe+3 iron salts & acid are liberated as effluents. These effluents need proper treatment to remove their toxic properties & make them eco-environment friendly. In this industry, the effluent is treated with a large effluent treatment plant where acid and iron is removed satisfactorily. After the treatment acid part is evaluated by the help of pH meter resulting generally 6 to 8 range.

The major objective of this theme is to make the monitoring & remove the remaining iron part in the ultimate water, which is discharged after effluent treatment, & resulting an efficient regeneration & reuse of that water.

Before electroplating the metal, the surface must be clean. Various acids such as HCl, HNO3 or H2SO4 are used for this purpose as picking liquors to remove surface oxides. The spent liquors contain residual acid and appropriate salts. Their compositions depend on treated metal objects. Very often they contain very harmful heavy metals. The components from spent liquor must be removed, recovered, or recycled for environmental reasons.

A neutralization procedure used traditionally for the effluent treatment is not always sufficient for their purification. From the viewpoint of both the environmental protection and economy, pollutants should be removed, whereas valuable materials can be recovered. The different techniques have been developed to purify wastewater from the iron and steel industry and to recover the components. The membrane techniques play a particular role in effluent treatments.

Pickling processing is a process through which scales & iron oxide residue on the hot rolled strip coils are removed. Mainly on the hot rolled strip coils, three types of iron oxide layers is observed a)Ferric oxide (Fe2O3) Upper Hardest Layer b). Ferrosoferric oxide(Fe3O4) c) Ferrous Oxide (FeO) Lower Most Layer. By removing the scales and oxide layers from the surface of hot rolled strip coils it has become suitable for Cold Rolling. As the oxide layers and scales are harder than the base metal, therefore it is necessary to remove these oxides and scales for further processing. A pickling line for steel strip is made up of pay off equipment, tension control equipment, strip centring devices, hydrochloric acid bath, water cleaning bath, hot blast drying & reels to rewinding the strips treated. Also, pickling lines are equipped with acid regeneration or wasted acid recycle system. The pickling line is used to prepare clean input materials for the consequent cold rolling steel strip production.

Preference of HCl over H2SO4 (Reasons)

Acid consumption with the regeneration process in place generally varies between 6 and 7 kg/ton but with the acid top spray process, acid consumption can be brought down to 4 kg/ton. This system can be added to all existing pickling plants as well as new plants, as it helps to reduce acid consumption.
Preference of HCl over H2SO4 (Reasons)

  1. HCl is cheaper as compared to H2SO4 and its handling is also safe.
  2. HCl is easily available as compared to H2SO4.
  3. HCl shows inertness to a base metal (reacts only with oxides and scales) but H2SO4 reacts with metallic iron & producing some insoluble precipitations of metallic sulphate & ultimately the overall reaction is checked.
  4. The Regeneration of HCl is easy as compared to H2SO4.
  5. When HCl is used Oxygen comes out to the atmosphere.
Effluent treatment process:

During pickling processing, several harmful chemical products aregenerated. Those are highly toxic to the human health &environment. Therefore, those need an effective treatment, whichcan remove their toxic properties in addition to make themeco-friendly. Several steps of the effluent treatment process are
discussed as follows:

Emission of discharged slug:
Chemical reactions of pickling are as follows:

  1. Fe2O3+8 HCl 4FeCl2 + O2 + 4H2O
  2. Fe2O3+6HCl2FeCl3 + 3H2O
  3. Fe3O4+8HClFeCl2 + 2FeCl3 + 4H2O
  4. FeO +2HClFeCl2 + H2O
From the above reactions, it is clear that Ferric chloride (FeCl3), Ferrous chloride (FeCl2) & Hydrochloric acid (HCl) are the primary products, which are generated during the pickling process. Those slugs are liberating out & stored in Neutralization Tank.

Neutralization:
Neutralization tank: There are 3 neutralization tanks in the effluent treatment plant. Firstly, the discharged slug of acid & iron is stored in the tanks. One generating pipe is liberated from each tank & linked to the lime tank1 & lime tank 2. A pump is attached to each of the generating pipes. So that the slugs from the neutralization tank pumped out to the lime tank1 & lime tank 2.

Lime tank 1 & Lime tank 2: In the lime tanks neutralization reaction takes place, as the resulting acid neutralized by lime (CaCo3)& soluble calcium chloride & carbon-di-oxide is produced.

CaCO3 + 2HCl CaCl2 + CO2 + H2O

Ca(OH)2 + 2HCl CaCl2 + 2H20