The waste liquids discharged during the acid bath process include sulfuric acid waste and hydrochloric acid waste. Large and medium-sized enterprises typically need to dispose of over 250 tons of waste acid liquid per month, which often contains approximately 4% sulfuric acid, more than 20% ferrous sulfate, and other metals.
Treatment of Sulfuric Acid Pickling Waste:
(1) Neutralization Method: This method uses limestone as a neutralizing agent to neutralize the sulfuric acid in the waste liquid. It produces insoluble calcium sulfate containing iron, which can be periodically discharged and stored. However, the ferrous sulfate in the waste liquid is not recovered. This method is currently less used.
(2) Production of Ferrous Sulfate: Some domestic manufacturers utilize waste acid liquid by pumping it into a treatment tank, adding a fixed amount of iron filings residue, and heating with steam to promote the reaction between sulfuric acid and iron to produce ferrous sulfate, thereby reducing the acid concentration in the waste acid liquid. When the pH reaches 6 and the density reaches 1126g/cm, the waste liquid can be filtered to remove iron filings and impurities before being discharged into a crystallization tank, where it is cooled to obtain solid ferrous sulfate. This method is simple and feasible and can produce ferrous sulfate as a by-product.
(3) Natural Crystallization and Concentration Crystallization Method for Iron Reduction: The acid wash solution that cannot be used online is cooled or evaporated and concentrated to precipitate iron in the form of ferrous sulfate heptahydrate. After solid-liquid separation, ferrous sulfate heptahydrate and acid liquid are obtained, and the acid liquid can be returned for use.
Treatment of Hydrochloric Acid Pickling Waste:
Hydrochloric acid pickling waste is produced in smaller quantities during the hot-dip galvanizing process, and factories generally use the neutralization method to treat hydrochloric acid pickling waste liquid.
