Spent pickling liquor treatment

The treatment of spent pickling liquor

Neutralization

SPL and SSL cannot just be discharged due to the environmental reasons described, therefore the fate of these effluents is usually neutralized with alkali substances to encourage the precipitation of metal hydroxides, primarily Fe and Zn.

Key emissions

The main concern related to water treatment are metals which are used as soluble salts. Liquid waste is generated during metal surface treatment processes such as degreasing, acid pickling, alloy baths or lacquering.

Recovery and treatment

Life Dime treats SPL and SSL, which are an environmental problem for the steelmaking process, hot-dip galvanizing and other metallurgical sectors such as iron or aluminium industries. Life Dime provides a low cost process to recover HCl and metals salts (Zn or Fe) contained in SPL and SSL.

The environmental impact of liquid waste from spent pickling solutions

Their highly corrosive properties are dangerous for the environment and collectors could have the following effects on the receiving environment:

  • – Hydrolysis of ferrous salts into ferrous-ferric hydroxides, insoluble in water, results in ocher sediment.
  • – The reduction of oxygen dissolved in water produced by the above reaction decreases the regenerative power of rivers.
  • – The proliferation of ferruginous bacteria and ferritic deposits affecs flora and fauna.
  • – High Fe concentrations in the solution discards waters to be used for food supply, household or industrial purposes.

The toxic nature of certain additives used in metal surface treatment solutions such as corrosion inhibitors and surfactants with harmful effects to the environment.

  • – Contents of non-ferrous metals such as Zn, Cu … in pickling/galvanizing baths.
  • – Gaseous emissions (nitrous and HCl gas) during the process.
  • – Liquid emissions due to subsequent washes.
  • – High salinity and organic load of effluents derived from the physio-chemical treatment of exhausted baths.

Waste generation in the treatment of metal surfaces

The main environmental issues arising from the surface treatment of metals relate to energy and water consumption, the consumption of raw materials, emissions to surface and groundwater, solid and liquid waste and the condition of the site when activities are discontinued.

The most concerning emissions to water include metals which are used as soluble salts. A number of liquid wastes are produced during metal surface treatment processes such as degreasing, acid pickling, alloy baths or lacquering.

Neutralization, besides being an economic burden to the metal industry, generates enormous disposal problems due to the high salinity of the treated effluent and the generation of large amounts of sludge that must be rendered inert prior to disposal at landfill sites. In addition, this procedure does not allow the recovery of any raw material or by-product.

Waste generation in pickling processes

Pickling or chemical descaling is the most common chemical procedure used to remove impurities such as stains, inorganic contamination, rust, surface oxides and other contaminants from metals surface, HCl is the pickling acid used to perform the chemical reaction.

The acid becomes more dilute and less effective with use. Once the strength of HCl has been reduced, it cannot be reused. It must then be disposed as schedule waste and refilled once it becomes too dilute to be effective.

Although different acids can be used as pickling acid, HCl is preferred for the pickling process because it is considered to enable a faster cleaning rate at normal temperatures compared to other acids used for the pickling process.

The final result of this pickling process is that all layers of impurities dissolves as ions into the acid solution. This pickling process creates large amounts of Spent Pickling Liquor (SPL) which contains the dissolved iron, chromium, copper, nickel and zinc metal salts as well as combined and residual free acid.

The corrosive nature and presence of residual acid and metals are the reasons why SPL is considered a toxic and hazardous waste (pickling acids: LER 11 01 05*).

Pickling is widely used to descale or clean steel in various steelmaking processes and it is also a crucial step in the galvanization process. Hot-dip galvanization is a corrosion protection process for steel, iron or aluminium in which the metal is coated with zinc to prevent it from rusting.

The process involves dipping clean steel, iron or aluminium components (after a pickling process) into molten zinc (which is usually around 450°C).

A series of zinc-iron alloy layers are formed by a metallurgical reaction between the iron and zinc creating a strong bond between the steel and coating. This technique requires enormous amounts of water during all its stages. It thus creates large volumes of wastewater loaded with total suspended solids and metals which are harmful to health and the environment (for each tonne of galvanized pieces 70 kg of SSL is produced).

Hence, the Spent Strip Liquor (SSL) is also considered toxic and hazardous due to the zinc that is added to the spent pickling mixture.