The waste liquids discharged during the acid pickling process include sulfuric acid waste and hydrochloric acid waste. Large and medium-sized enterprises generate over 250 tons of waste acid liquid per month that requires disposal. These waste liquids often contain approximately 4% sulfuric acid, over 20% ferrous sulfate, and other metals.
Treatment of Sulfuric Acid Waste Liquid:
(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 certain amount of iron filings residue, and heating with steam to promote the reaction between sulfuric acid and iron to produce ferrous sulfate, 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. Upon cooling, solid ferrous sulfate is obtained. This method is simple and feasible, allowing for the production of ferrous sulfate as a by-product.
(3) Natural Crystallization and Concentrated Crystallization Method for Iron Reduction: The acid bath 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 reuse.
Treatment of Hydrochloric Acid Bath:
The production of hydrochloric acid bath waste liquid is relatively low during the hot-dip galvanizing process, and factories generally use the neutralization method to treat hydrochloric acid pickling waste liquid.
